Avatar: The Last Airbender/The Four Nations/WMG

The reason why there aren't any Air Nomad Nonbenders is because they would have made the first attempt to Take Over the World.
Since the logical Fantasy Counterpart Culture for Air Nomad nonbenders are Flying Mongols (Tibetian Buddhism spread like wildfire through the Mongolians after they were driven out of China by the Ming Dynasty) or mounted archers of some sort, it must follow that they must have tried for world domination at least once. Knowing the Mongol's historical attempt at it, they would have been very successful, considering their obvious advantages (Ba Sing Se? No match for those Sky-Bison mounted raiders with composite bows and gliders). However, it would have been reasonably more interesting to have the Fire Nation, with it's Industrial-level technology; be the first of the four peoples to try to take over the world. There is also the elemental symbolism: Air has traditionally been seen as one of the "good" elements, while Fire is usually the opposite (and no, I do not mean any malice in that statement). In short, the Fire Nation was the much better choice for "attempted world dominator", and This Troper has made his peace with that.
 * It's possible that this happened at some point in the past. For a world that's been at peace up until the Fire Nation invaded, they seemed to be pretty well set up to, you know, fight eachother.
 * True. Also, it's well known that the steppe nomads were great archers in their day. Could this be replicated in the Avatar World too? Maybe the Yu Yuan Archers were remnants of the Nonbender takeover attempt who managed to settle in the Fire Nation and keep their archery skills, making them useful when the Fire Nation decided to have imperialist ambitions. Just a thought...
 * Well, we know for a fact that in Avatar Kyoshi's lifetime, there was a conquering tyrant who may have had designs on world domination who originated in the Earth Kingdom. Considering how long the Avatar has been around (Roku implies that there have been at least a thousand Avatars, which would suggest a period of tens of thousands of years), it is likely that the history of the world means that the four nations have not always been in the state that they were when the war started by Sozin began. Indeed, the Word of God has said that the spirituality of the Fire Nation by the end of the war was low enough that the ratio of Benders to Non-Benders was lower than any other nation (in contrast to the 100% Airbender Air Nomads), so it has probably waxed and waned for the other nations. Also, "The Avatar and the Fire Lord" implies that the prosperity and unity of the Fire Nation in Sozin's time was a fairly recent development. We can probably assume that there has been war between (or within; nation is not necessarily the same thing as state) the different nations throughout the Avatar's history.
 * A water-empire also seems likely not too far back in the world's history, judging by the distinct populations of physically and mystically similar people at opposite ends of the world. With their more fortified and sophisticated architecture, the Northern Water Tribe probably represents the core of the former empire; the Southern Tribe and the Swamp Tribe are remnant of colonies.
 * Maybe the Air Nomads are like the Levites of Ancient Israel. The ancestor of that particular tribe (Levi) went on a killing spree. Therefore, while all the other Israelites received a tract of land, the Levites were scattered all over the nation. However, they were given the task of being a priestly class. Though they could never really have land as a tribe, they were the most spiritual people of the land. So the Air Nomads tried to take over but failed. They were forced to be scattered among the nations, doomed to forever wander the earth (Frankenstein or Prometheus -like). But to ensure no one looked down upon them, they became the spiritual leaders of the whole world.
 * It should be noted that there are airbending weapons. Not just tools like Aang's staff, which can be re appropriated as weapons, but there is at least one airbending weapon mentioned. It's in the episode Sokka's Master, Aang mentions that there is a kind of weapon that's usually just a hilt until you bend air through it and make a blade. Seems like an odd thing for a nation of pacifist monks to have. So it's not unreasonable to assume that sometime in the past there was some form of airbending civilization or sect that was willing to use deadly weapons.
 * Also, the nations have proven to change over time. The Sun Warriors apparently preceded the Fire Nation as... well, the nation of Fire. And there are a lot of ruins in the Earth Kingdom.

Aang is not the last Airbender...
...or at least won't be if he ever starts having children.
 * Ty Lee is a descendant of the Air Nomads. Ty Lee's extremely acrobatic She Fu sometimes seems to be more than just cartoon and/or action physics can explain. She must be (unconsciously) airbending to assist her acrobatics.
 * It's worth noting that she has brown hair and grey eyes (like most Air Nomads), rather than the Fire Nation's usual black-and-gold. Also, Ty Lee is the only person seen in the Fire Nation with round-faced features almost exactly like Aang's.
 * A fanfiction of this theory can be found here
 * Rewatch the episode The Headband. There are lots of Fire Nation children with rounder facial features similar to Aang and Ty Lee.
 * Most of those children are Aang's age or younger, and so can be expected to still have child-like features. Fire Nation teenagers and young adults tend to have much sharper features.
 * Alternately, the island in The Headband is the one that Ty Lee's from, and they're all secret airbenders.
 * Aang will not be the last Airbender because Karma itself will fix the reincarnation cycle. People who currently seem to possess no Bending abilities, but share a strong affinity with air (like acrobat Ty Lee, knife-thrower Mai and the colonists of the Northern Air Temple) will spontaneously manifest airbending, and feel strangely drawn to the old Air Temples, setting up societies there, and learning airbending techniques from the only living master on Earth--Aang himself.
 * It's heavily implied by the Finale. If he can take off bending abilities, why not give?
 * I like this idea, it gives a nice beginning to a new air nation. I'd like to add the kyoshi warriors to the list though (they don't seem to have any earth-benders and fight with fans)
 * Reports of the Air Nomad genocide have been greatly exaggerated. As noted on the It Just Bugs Me page, the temples house kids and elders, while the rest of the nomads travel between them. Some of them must have survived, banded together, and had children. These descendants were dedicated to finding the Avatar and supporting his cause, and made like-minded allies in the three other nations. This society would have to remain secret, of course, so it would come to be known by the name of it's symbol... "The White Lotus."
 * Alternatively, Aang is sterile. After all, being frozen in ice for so long could certainly cause some health problems, and since since he's not having trouble breathing, seeing, hearing, walking, etc, those problems are most likely to crop up in the one area that hasn't had to work in that timeslot.
 * Maybe not sterile, but not particularly fertile either. It seems that he and Katara are destined to have only one child.
 * Aang is not sterile. He was confirmed as having three kids with Katara.
 * I don't think he was actually frozen. More like he entered a state of suspended animation, and just happened to create a sphere of ice around himself because it was the best protection he could get at the time.
 * Also, that's not really how illness works.
 * Jossed by the new series where Aang's son is teaching the new Avatar airbending.
 * My theory is, that most or all of Aang's children will be airbenders, and most/all of their children, and most/all of their children, until they reach the point when they're all so distantly related they can make babies with each-other without worrying about genetic diseases, and then the Air Nomad culture will be born again.
 * Jossed by the new series. Aang had three children, only one of whom (Tenzin) is an Airbender. However all of his children (so far) are Airbenders.
 * People "learned" how to bend from various sources. My theory is that anyone with bending potential can learn to bend any element -- but only one element; once you've learned one, you're locked. And growing up around benders, seeing their techniques, being immersed in that element, tends to predispose one. But Aang can find untrained benders and teach them airbending.
 * By this logic, Appa could also teach airbending, seeing as he's a Sky Bison.
 * Unfortunately though, being the last Sky Bison, once he dies there will be no natural source of airbending for humans to potentially learn from, which makes it even more important for Aang to have descendants and get some people from the other nations to integrate into air nomad culture (and even if only a blood descendant can learn airbending, his children will need people to live and have children with) and try to learn airbending themselves. Of course, this will cause a caste system to develop among the new air nomads, between blood descendants of Aang and everyone else.
 * Jossed. Word of God confirmed Aang found a whole herd of wild sky bison and they're repopulating.
 * Also, Avatars live for 1-2 centuries typically. Hence, assuming they don't die early deaths, It will be something on the order of 4-8 centuries before the next Airbender Avatar is needed anyways
 * Every Avatar is an airbender.
 * The show occasionally notes that Airbending, Firebending, Earthbending, and Waterbending originated from ancient peoples studying elements of nature, such as skybisons, dragons, badgersmoles, and even the moon, respectively. They watched how each animal / the moon manipulated an element, and developed methods to replicate it. Following that trail of thought, all four bending styles could be said to have been manufactured by humans watching nature. So a skybison like Appa teaching a human to Airbend isn't too unbelievable, seeing as it's been done before.
 * Tantalizingly, this would imply that any human with bending potential could, with enough discipline and an observable subject, learn to bend anything.
 * Do they even need an observable subject? Toph invented Metalbending simply by realizing that metal, as Iroh said, "is simply earth and rock refined into a stronger form."
 * The Guru mentioned that the differences between the elements are illusionary, and that they're actually not inherently separated (in that scene where Toph learned to metalbend). Maybe benders aren't even "locked" when they learn one discipline, but Aang is the only person who knows he can learn them all and who has enough talent to learn each of them in less than a life time.
 * I heard that it's Word of God that bending is determined by personality, upbringing, and spirituality rather than genetics, so it's not impossible for a person not related to Aang to pick up airbending.
 * Which would be exactly why Ty Lee and Mai cannot Firebend. Mai is too "cold," and doesn't express emotions much, which is central to firebending. Conversely, Ty Lee is too "warm," as the Firebending technique used at the time foucused on anger rather than positive emotions.
 * It seems upbringing also includes the location where one was raised. Otherwise the hippies from the cave of the two lovers would probably have been airbenders. And the sandbenders would have a large population of firebender births. So, northern air temple seems most likely to bring forth new airbenders.
 * It is possible that over the centuries many of the airbenders interbreed with other nations. The fire nation and earth kingdom seem to be the most common. With Aang's help many of these people will start airbending. Teo for example looks like an airbender and likely has some air nomad in him. His father also likely is a decendent of an air nomad.

Aang will create a new generation of Air Nomads.

 * On his first try, he was able to
 * Unlikely: I bet
 * Actually and it would probably be easier when used on someone who's personality is similar to that of an air nomad, like the people living at the Northern Air Temple.
 * Agreed.
 * Bending one element or another isn't genetic. All Aang has to do is find lots of war orphans and teach the most promising ones Airbending. Aside from an inborn talent of bending strength, the only thing necessary to learn bending is a teacher and starting early enough to make the bending philosophy a part of your life.
 * Well if that's the case, why can't all benders bend all elements?
 * Bending disciplines aren't just skills, they're ways of thinking. The reason one type is associated with each nation is that over time the dominant element among each group has influenced their culture, giving rise to societies that promote the personalities and worldviews associated with it.

Aang is the Last Airbender, and his ultimate destiny is to usher in the end of the Four Nations and their individual bending styles.
Aang and his multicultural group of friends are a sign of the world's future. The national boundaries between nations will break down, spurred both by affects of the war and by industrialization, creating a greater intermingling of peoples. Look at Team Avatar's romantic relationships, where Aang (Air Nomad) and Katara (Water Tribe) are mixing just as Sokka (Water Tribe) and Suki (Earth Kingdom) are. The boundaries between the Earth Kingdom and Fire Nation have been blurred by the war and colonization efforts. With technology like airships, international travel and trade will be made easier, resulting in greater intermarriage. Since Word of God has it that Aang is the last airbender, the one thing he can pass on to future generations is the knowledge of a bending style you don't need to be born with -- Energybending. Just as the world was once unified as one people who used energybending, so it will be again. The divisions between the nations and the four bending arts are said to be artificial by the like of Huu and Guru Pathik. Aang, and the final three Avatars after him, will use the knowledge of energybending to reform a newly globalized world into a single nation and people.

The separation between the four nations is actually a socio-religious one, rather than genetic.
The Guru makes a big speech about how everyone in the world is related when it comes down to it, and the Word of God has said that religion is more important when determining who can be a Bender. Thus, the distinction of the four races is totally psychological, with the Avatar being the only one who can mentally overcome said blocks and master all of the best aspects of the four cultures combined. This would also mean that there could be no "Last Airbender" in the same way there could be no last practitioner of carpentry or of Judaisim, unless the teachings and concepts of such things were totally wiped off the face of the planet. Of course, it doesn't appear that any of the Airbenders' teachings were written down, so Aang is still the Last Airbender; if he goes without teaching anyone or making a record, the teachings go with him and the next Avatar can't learn airbending.
 * He might be able to figure out a little air bending from first principles. Someone must have.
 * Nope. All bending was taught, remember? The Air Nomads got it from the Sky Bison.
 * And where did the Sky Bison get it?
 * The sky.

The Sozin's Comet day Air Nomad genocide was not 100%
As suggested earlier, not all the Air Nomads were killed when Sozin's comet was overhead. They all couldn't have been in the temples at the time of the attack. Basically, it was like Order 66. Most of the population was destroyed, and the scattered remnants were ruthlessly hunted down, but some savvy Air Nomads hid themselves and would not openly display their Airbending abilities. The subsequent 100 years dwindled the population because of miscegenation, the inability to practice or train new benders, and the need for secrecy, but aside from Aang, the last Airbender likely died only a couple of decades prior to the series start rather than shortly after Aang got frozen.
 * While there still are likely people with Air Nomad liniage. There are no other Airbenders left, which was the group Sozin was hunting on account of the Avatar. Once the Avatar "disappeared" like he did, the civilian Air nomads were probably incorporated into the Fire Nation.
 * They're nomads. Nomads do not sit in one spot, it's kinda the definition of the word. The four great temples represent permanence amongst the Air Nomad people, but as Aang notes, they don't have a military, which means, presuming the term 'nomad' wasn't just used because it sounds cool, they were decentralised and spread out, making the population of the monasteries a minority. When news of the destruction of the temples spreads, any sensible nomadic people are going to swap their robes for local gear and spend the next However Long they can integrating into the society in which they find themselves, or at least not clearly identifying themselves as 'air nomads'. This theory means Air Nomad peoples could be spread out across any of the other communities. If one presumes all Air Nomads were Air Benders, and we accept the narrative that Aang is the last air bender, then yes, he must be the last of the Air Nomads as well. But if they all lived in one of four spots and didn't move around, they're pretty sucky at being Nomads. Presuming that the monasteries represented the Bending population of the Air Nomads, though, the two story elements dovetail decently and allow for oddities like yes, Tai Lee, with her Air-Nomad-ish features.

The peaceful, happy final scene in the series is only a brief respite. Things in the Avatar world are still very unstable and decidedly not peaceful.
The Earth Kingdom seemed to be a loose federation of smaller kingdoms and city-states, not really a true empire. After Zuko has the Fire Nation pull out its troops, many of the larger city-states under Fire Nation rule for so long find themselves with a power vacuum. Not to mention that the nominal head of the whole shebang, the Earth King, seems to be uninterested in returning to his throne. All of that is ripe fodder for power struggles and widespread civil conflicts. Then there's the whole matter of the Fire Nation itself. Ozai wasn't the only power hungry, sociopathic bastard in the Fire Nation (as seen with the many smirking generals when Ozai gave Zuko his scar) and I doubt many of them would recognize a ruler who has a history of being exiled and betraying his father (essentially, a subject who probably would have been executed had he been a peasant or a lesser noble and not related to the Firelord). They may use the post-war confusion as an excuse to either carve out their own little fiefdoms in the Earth Kingdom or maybe even start a coup. In short, our heroes are still gonna be very busy for some time to come. ""In Gondor the King Elessar now ruled, and in Arnor also. In all the lands of those realms of old he was king, save in Rohan only; for he renewed to Eomer the gift of Cirion, and Eomer took again the Oath of Eorl. Often he fulfilled it. For though Sauron had passed, the hatreds and evils that he bred had not died, and the King of the West had many enemies to subdue before the White Tree could grow in peace.""
 * This is a traditional ending for fantasy epics: even after the great evil overlord is defeated, the heroes still have years of mop-up left ahead of them. Its what they do in between the happy ending and dying of old age, rather than being bored for decades. As an example, from the appendix of Return of the King:


 * However, things aren't necessarily as bad as all that. While the Earth Kingdom is politically disorganized, that will only act to prevent major wars for the foreseeable future. Wars of revenge vs. the Fire Nation have to deal with the limitation that you need a navy to reach them, and the Water Tribes aren't interested in revenge. (Or, rather, while individual Water tribesmen might still be PO'ed, the Northern Water Tribe has no naval expeditionary force and the Southern Water Tribe's chief and heir are both firmly behind the Avatar's peace plan.) Ambitious bastards in the Fire Nation have to deal with the fact that the Fire Lord has overwhelming support from the commons, who are from all appearances simply ecstatic that the war is over and Ozai's iron boot is off their neck. And looming over everything is the fact that the Avatar and the Order of the White Lotus have both spectacularly and publicly proven themselves capable of wrecking armies all on their ownsome. So, while there's definitely going to be some mop-up left to do, there's really nobody left who can significantly challenge the Avatar/Firelord/White Lotus/Water Tribe alliance of peace.
 * Good points, but it need not necessarily be wars of revenge against the fire nation that would cause unrest. And while the politically disorganized state of the Earth Kingdom might prevent major wars against foreign powers from breaking out, history (hell, even current events) shows that political disorganization is a breeding ground for internal power struggles and civil unrest. The Earth Kingdom being as large as it is, it is perfectly feasible that some ambitious bastards in the under newly-freed city-states would try to take power, legitimately or otherwise. Then there's the Dai Li, who were "banished" by Azula. Which means they might still be a viable power, and might try to install another puppet Earth King. Given that they publicly supported Azula, such attempts would not go over well with the population. As for the Fire Nation, even with Zuko having popular support, even just a small number of ambitious Fire Nation commanders with their own private forces (the Fire Nation military organization always struck me as similar to the Romans just before the rise of Augustus, with armies loyal to the generals and not the nation. Zhao and his troops come to mind) can still make a great nuisance of themselves. Especially if the rougue commanders are smart enough to use asymmetrical warfare than direct confrontation. All that combined is gonna be a huge headache, and as powerful as the Avatar is he can't be everywhere at once.
 * Asymmetrical warfare? "The guerrilla must move amongst the people as a fish swims in the sea." Attempting guerrilla warfare without the support of the populace is a fancy way of saying 'You're a bandit'. Bandits can be damned annoying, but they aren't overthrowing anything. As for the Dai Li, This Troper wishes them the best of luck vs. Grand Master Iroh of the White Lotus, who is maybe-not-so-coincidentally hanging out his shingle in Ba Sing Se these days. And while the Avatar can only be in one place at once, now that he has full mastery of his powers he's also invincible. The only thing on the planet that could conceivably stop him is the entire Order of the White Lotus assembled, and they're on Aang's side. What idiot is going to start a war of conquest when he knows that his lifespan is "However long it takes the Avatar to clear his calendar and get around to stomping me + 10 minutes"? Aang is the guy who owns nuclear ICBMs when nobody else in the world does -- you cannot make open war on him or anything he's protecting unless you're a complete idiot, and the reason covert warfare isn't going to work is described above. (As for killing him in his sleep, the only assassins in the world who might have even a remote chance of success work for the Fire Lord now.)
 * There is the whole "Kidnap-Avatar's-Loved-Ones-To-Force-His-Compliance" gambit. Problem is, assuming it does not backfire, there are some practical problems: Aang has no living family; his love interest is easily one of the 10 most fearsome waterbenders on the planet; and his True Companions... the "easiest" target is the student of one of the greatest warriors alive, the lover of one who is no slouch herself (not to mention the leader of the Kyoshi Warrors), an up and coming member of the aformentioned Order Of The White Lotus, the heir apparent to the chiefdom of the Southern Water Tribe, and the former boyfriend of the new moon spirit.
 * Even Sozin had to wait for Avatar Roku to die from his own overconfidence before he could embark on Plan World Conquest, and Sozin wasn't operating under anything remotely like the handicaps that hypothetical a rogue Earth Nation city-state or Fire Nation general would be. One Avatar in the fullness of his power was a sufficient nuclear deterrent to keep the entire Fire Nation, united and at the height of its power, in check for decades. And if Aang happens to get hit by a meteor? The next Avatar will be born in the Water Tribe. Most likely the Northern... and, good luck cracking those defenses. The Air Nomads were the least militarized of all, and even then Sozin needed the comet, and the full might of the Fire Nation at its height.
 * Actually, overwhelming support from the commoners might not be a given at this point. It was revealed in "The Headband" and in "The Ember Island Players," that Ozai had been building a cult of personality around himself, meaning that the society of the Fire Nation has been built around grooming people from childhood to adore him. Given the way that most of the people responded to the end of the play put on by the Ember Island Players (that is, wildly cheering at Zuko and Aang's defeat), this indicates that they might not be so happy with the end result of this whole conflict. Also, given how long the Fire Nation has been at this conquest and colonization thing, some people have presumably been living in the Earth Kingdom colonies for generations now and would be too happy with being told that they have to pack up and go back to the Fire Nation. This also has the potential to result in extra conflict as Earth Kingdom residents who want their ancestral land back clash with colonists who can now claim that the same land is "their" ancestral land and you can add a layer of civil strife to all the other potential conflicts that might break out.
 * Ambitious militaristic fanatics might not necessarily be rational. They may well convince themselves they actually stand a chance when they don't. They might all get stomped by Aang may still have to do a lot of "clearing the calendar + 10 minutes" to do, keeping him busy for some time.
 * Azula's breakdown might not be permanent. If she recovers and some group (sympathetic to her, or just wanting to use her thinking erroneously that she's still insane and easy to manipulate) breaks her out of whatever prison they're keeping her in, she could still become a dangerous threat. Not a direct match for Aang and his allies, but still potentially quite dangerous. (Keep in mind that as a teenage prodigy she may not have actually yet reached her full potential, she could possibly get even better at firebender with additional training, though the same can also be true of the Gaang.)
 * Azula is the single biggest threat to world peace in the Avatar world and the Gaang were fools to leave her alive. She has the power to command and an aura of authority that would gather disgruntled nobles and disenfranchised commoners to her cause. If the Fire Nation is asked to forfeit all properties gained during the war, we have people being relocated from the homes they've lived in for generations (see Israel and Palestine for how badly that can go). Governors like Mai's father are suddenly out of work and displeased that their Fire Lord didn't fight for them. The biggest problem is the military industrial complex the Fire Nation built up. A large portion of their citizens work in the military and if the war is over, it will be hard to justify paying them their usual salaries. We've seen no indication that the Fire Nation as a whole is tired of the war. This will be worsened if the Earth Kingdom and Water Tribes demand reparations. The commoners of the Fire Nation will care less about the "harmony of the four elements" and world peace if they're taxed to repair Earth Kingdom settlements. All Azula has to do is beat the drum of Fire Nation Nationalism that 100 years of war helped build. If Zuko bungs this up and tries to appease the Avatar and the other nations before securing the Fire Nation, he and Mai will be first against the wall when Azula leads her People's Army of Glorious and Harmonious Cultural Revolution to do a Lenin on Zuko's Romanov.

The four Nations are supposed to represent the four basic personalities.
This actually works, if you think about it. Air Nomads are all basically Sanguine: fun-loving, playful, non-confrontational, and personable. The Fire Nation is definitely Choleric: hot-blooded, stubborn, ambitious, dominating, and charismatic. The Earth Kingdom could be considered Phlegmatic: calm, dependable, not given to outbursts of emotion (Toph is probably the exception here). The Water Tribes are, of course, Melancholic: emotional, perfectionists, sensitive, and strategic.
 * You're certainly correct, but it's a strange way to go considering that the order is usually Fire: Choleric, Water: Phlegmatic, Air: Sanguine, Earth: Meloncholoc.
 * Which does fit the benders and most secondary characters much more accurately. Even Aang, with the exclusion of the times that he's actually acting his age.

In the the Fire Nation Authority really does equal asskicking.
Fire is the Element of ambition, so those who have the most ambition, generals, princes, and Firelords, kick way more ass. Ozai was such a fire-bending monster because he was trying to destroy an entire civilization. Iroh managed his greatest feats of badassery when he was his most ambitious; taking Ba Sing Se and such.

The Noble (and Royal) Fire Families are Air Nomads
Not all nobles, but some do display Air Nomad qualities.Ty Lee has been argued to be an Air Nomad because of her hair and eye coloration and eye shape. She also seems to have the free spirit and spirituality of a nomad (she can manipulate chi/chakra and read auras and is quite a Twinkle Fingers).

Ta Min, wife of Roku,has the rather noticeable eye color and shape of Aang.Iroh mysteriously said something about the Royal Family being more than meets the eye (Transformers?). And Ty Lee said on the Ember Island beach to Zuko when he was confused "But I know you," though little evidence shows this. If Ty Lee and Ta Min descended from the Air Nomads--maybe the same clan, that could explain the mysterious quotes. That would mean Zuko and Azula would be part Air.

And the Airbender tattoo may still be a part of their culture. In the pictures of Ty Lee and Ta Min,their foreheads are never fully exposed. Maybe just the side tips of the arrows remain as a reminder of their culture.

Further proof: Zuko cuts air with his swords. Just Kidding.
 * Except the only people who get arrow tattoos are Airbending masters. But it's not unlikely that Ty Lee and Ta Min might have had some Air Nomad ancestry.

Pathik is the last remnant(or is he?...) of an older nation
Long ago, a fifth nation existed: the Spirit state. Their specialty was manipulation of the chakras and the spirit world, and their culture was based on that of india. Probably worshiped elephant-cows at one point or another. They must have been destroyed by the Fire nation at some point.
 * Wouldn't Aang know about them then, since he seems to know at least a bit about everything else, and he was around before the fire nation attacked? For that matter, wouldn't everyone else? It's only been 100 years.
 * It's also possible that they split off from the Air Nomads by settling someplace, just as the Sun Warriors split off from the Fire Nation yey before Aang's time.

Foggy Swamp used to be a Tenochtitlan-esque city, and the capital of a vast waterbender empire.
The old Water Empire stretched from from one pole to the other, thus explaining the presence of near-identical cultures at opposite ends of the planet. And in keeping with the Aztec theme, they most likely practiced human sacrifice, thus causing the Avatar to destroy their civilization. With their empire gone, they couldn't maintain their capital and it was consumed by the swamp.
 * Presumably they fought the Sun Warriors at some point.

The Southern Water Tribe is mostly descended from cast-out female waterbenders and their closest friends and immediate families.
The Northern Water Tribe refuses to acknowledge the existence of skilled femal waterbenders, or train them at all, while the Southern Water tribe's strongest benders (and the vast majority of their benders shown onscreen) are female. It's kind of obvious when you think about it. The reason it's more likely this than the other way is that the southern water tribe is less sexist (there were male waterbenders shown in the Fire Nation raid flashbacks) and that the Northern Tribe had a fortress whereas the Southern tribe had individually constructed homes (not proof in itself, but strong evidence).
 * In the puppet master you see that it was once like the north, perhaps they were allowed to because of the war. They learned not to be sexist after this, note no water tribe warriors were girls.

Even before the genocide, there were no Airbender females. Avatar Yang-chen was the last one.
Instead, Air Nomad babies were genetically engineered in test tubes. This is how they manage to stay bald even without shaving, and all have Airbending powers. It also allows the monks to keep celibacy vows (Do Buddhist monks do that?) As to how this is even possible in not very technologically advanced world, well... A Bender Did It.
 * Actually, if you watch "Appa's Lost Days" and the one after it you can see female Airbenders.
 * But, Aang has hair in the beginning of Book 3.
 * The Airbender females were in the East and West temples, the men in the north and south.
 * They don't have genetic engineering.
 * Tibetan Buddhist monks and nuns are celibate, as are most Buddhist monks and nuns. The exception is Japan. However, Aang himself.

The Airbenders that survived escaped to the mesosphere.
Seriously. The mesosphere is really windy and has nothing in it (except a couple ducks). That leaves loads of room for the airbenders to live and practice airbending.
 * Food? Water?
 * Aforementioned ducks and clouds!
 * They sent the sky-bison water cloud to the Gaang in the desert, providing much-needed spiritual and hyrological nutrition.
 * Finally we can die!

By the end of the series, most of the Fire Nation has lost it's Firebending prowess.
For the same reason Zuko did in "The Firebending Masters." Civilians and those who still support Ozai do not have this problem, and it may not be an issue for some soldiers, but this is the general rule.
 * IIRC, Zuko lost his firebending because it had been based on anger and his obsession with capturing the Avatar/honour/his father, which took a very different direction following his proper Heel Face Turn. Without that drive to fuel his abilities, he could not firebend. Only a few firebenders showed that level of obsession or dedication associated with Ozai's violent manner; Zhao & Azula (probably forgotten others). The fact Zuko found another source shows that there are different ways to firebend, and we're shown numerous times that technique has a lot to do with it not just the source of the power. There are variations, different frames of mind - look at Jeong Jeong who is well aware of the risks, or Iroh who was inspired to look to other bending disciplines to augment his own. The events of the end of the series, while undoubtedly a massive upheaval for the Fire Nation, would not necessarily have the same blanket effect on everyone in it regardless of alliegance, because like Zuko and Iroh they may draw their firebending from other emotional/spiritual sources.

Hama's prison was ATLA's equivalent of Unit 731.
Why else would the Fire Nation bother keeping all those waterbenders alive? Why didn't they just kill all of them, like they did with the airbenders? And it's not because it's a kid's show and they needed Hama for a plot device, nope.
 * Also note the followling sentece from the other wiki: Unit 731 was based at the Pingfang district of Harbin, the largest city in the Japanese puppet state of Manchukuo (now Northeast China). To think Hama wound up as the puppet-master afterwards...it's most probably completely coincidently and absolutely unrelated but at 2 am it was enough to let a chill run down my spine...

The Order of the White Lotus was not founded by humans and is responsible for the creation of the Avatar cycle.
Let's step back to the infancy of human society, perhaps at the time when humanity has begun to take their first steps away from a unified spirit-bending society into the four nations and the four elements. The forces of the world allowed this, and even helped guide it (this is how people learned bending from certain animals; Dragons encouraged the creation of firebending, and so on) but divisions in the world meant there had to be a balancing force. The oldest and wisest Lion Turtle decided to create this force, calling the greatest Dragon, Flying Bison, Badger Mole, and the moon spirits to watch over the developing nations and keep them in balance to make sure these changes did not cause a disaster.

However, the Order's guidance was unable to prevent a war from breaking out as humanity drifted further and further apart, and while the Order quickly put a stop to it the first time, they realized the human race needed someone to carry on after they were gone, human forces to work against chaos and war. While the other members of the Order began to select human pupils from the four societies to make a new Order of humans, the oldest Lion Turtle took an alternative route, discovering an orphan child of indeterminate origins in his wanderings and taking it under his wing, raising the child independently of other humans. As the Lion-Turtle suspected, doing this allowed the child to have limitless bending potential, reviving the art of spirit-bending among humans. Taking his student to the other members of the Order, the Lion Turtle asked that each of them pass on their knowledge to the child. Thus, the first Avatar learned all five forms of bending from the oldest and wisest beings in the world, and became such a potent spirit-bender that s/he became capable of reincarnating in a cycle and channeling the spirit world's power to enter the Avatar State.

Content with the existance of a spiritual balancing force in the Avatar and a human Order of the White Lotus to act as a political balancing force, the original Order withdrew from human affairs and slowly died off and were forgotten...

Until the Lion Turtle encountered the most recent incarnation of his first and last pupil.

There was a second group of Air people
Word of God says all Air Nomads where Air benders, maybe that's because the non Airbenders weren't technicaly Air nomads. When a baby is born in an Air Village it's tested for bending potential, those with the potential are taken by the monks to become Air nomads, the rest just lived normal lives.
 * Seconded because this sounds pretty fricken reasonable. XD

The Yu Yan Archers are really Airbenders.
The Yu Yan Archers split off from the Air Nomads because they (the Archers) wanted to fight and/or hunt. Notice how they didn't have any trouble keeping up with Aang while chasing him (they could "run like the wind"), and Airbending could account for their accuracy. There are examples of other bending subsets (Fire->Lightning, Water->Swamp/Blood, Earth->Sand/Metal), so it's not a stretch.

Agni Kais aren't done shirtless for freedom of movement or because fabric is more flammable than human skin.
The custom was initially promoted by the Fire Nation government as a way to encourage firebenders to work out more. It's not enough to be a better firebender than your opponent - you also need nice abs.
 * Nice Theory, but if this is so, why didn't Azula go topless for her Agni Kai against Zuko? It seemed rather important enough an event to follow all the rules and customs.
 * PS While the WMG is interesting, lets just be thankful that Lo and Li are NOT firebenders when ordered to Agni Kai or it was mercifully left unshown if they were.

Air Nomad children who couldn't bend were adopted into the other nations.
All the Air Nomads were Airbenders, but not everyone is born with bending ability. Children who couldn't airbend, and therefore couldn't be part of Air Nomad society, were put up for adoption in the other nations. This would explain why people other than the Air Nomads have gray eyes. A few of those children's descendants, such as Ty Lee, exhibit latent Airbending ability. Aang will use these people to rebuild the Air Nomads.

The day of the comet is a holy day to airbenders
So holy in fact, that all the airbenders gather at the temples to pray/meditate/perform a ceremony. Thus explaining why all the nomads were in one place.
 * Why would the comet mean anything to airbenders? It's a firebender thing.
 * Maybe they gathered in the temples for protection from firebenders, not realizing that the Fire Nation was planning genocide.
 * Perhaps the day of the comet just happened to be an Airbender Holy Day. It could've occurred on the Autumnal Equinox, for example.

Aang is not the last airbender during the time that A:TLA takes place.
This tropers original guess was that people of differing nationalities in the Avatar world couldn't have children, because their genes were incompatible, which is why we see no hybrids (half one nation, half another), but this is Jossed by [[The Legend of Korra, with Aang and Katara's son, who has airbending ability.

Instead, there are hybrid children all over the place, but any of them that are born benders can only master one element: the element of their father, or the element of their mother. Airbenders mingled with the other societies, so there are children of airbenders in all of the nations. The descendants of these children will not necessarily know of their Air Nomad blood, but they all have the potential to airbend, even if some of them haven't realised. Some of them have however, and hide this ability because they fear that what the Fire Nation inflicted upon the Air Nomads will happen to them if they are caught.
 * This theory works well with the "Ty Lee Is An Airbender" theory...

The death penalty is not legal in the Fire Nation
Ozai's speech immediately after the eclipse implies that banishment was the normal punishment for traitors. And there were no witnesses to his attempt to electrocute Zuko, because he'd ordered the guards to leave the room. If he was permitted as Firelord to execute people, why would he make sure he did so in a situation where no one else knew exactly what happened? He could have always claimed after the fact that it was another Agni Kai, or self-defense. When Mai and Ty Lee betray Azula, the worst punishment she can order is to throw them in prison. All this seems to indicate that killing as a punishment for crimes is not permissible in the Fire Nation, though it's still apparently OK to kill people during an Agni Kai, or to kill enemies in a war. For extra irony points, it was Sozin who banned capital punishment, as part of the Fire Nation's modernization program.