Avatar: The Last Airbender/Avatars/WMG

The first few avatars created the 4 nations in a very ancient world
The avatar spirit perhaps appeared first when people moved on from Energybending to bending the 4 elements. It's likely that the 4 nations did not exist then and there was only one nation earlier. It's not too much of a stretch to assume that just as Aang and Zuko decided to create Republic City as a melting pot for all benders and non-benders to live in balance, the first few avatars split the world into 4 nations based on the 4 elements. It's likely that since the Avatar has been reincarnating for almost a thousand lifetimes (or more), human society is hundreds of thousands of years old - the old paradigm of separating different people of different elements and cultures makes real sense then.

That also means that in a way, the avatar is responsible for practically everything that shaped the world and all the consequences that came out of it. Hence the need to keep reincarnating.

The Avatar's teachers are reincarnated with him/her
Something that always got to me was that Aang could only learn from certain people. He starts out learning from some old master (Pakku, Yu, Jeong Jeong) but most, if not all, of what he learns come from a kid closer to his age (Katara, Toph, Zuko) Why? The reason for everything in this show: Because Destiny Says So.

The old masters were great teachers, but not for the Avatar: Pakku treated Aang like any other student (which sounds fair until you realize that Aang does need special attention since he has to master the elements much fasster than other students do), Yu didn't even know that Aang was the Avatar until Toph's parents sent him to track her down, and Jeong Jeong couldn't teach him becuase of the circumstances.

So just who Katara, Toph, and Zuko reincarnated from? My guess is that they follow the same Avatar cycle. In "The Guru" it is implied that Karata is the reincarnation an Air Nomad (whoever formally taught Roku). It also was pretty much stated that Toph was destined to be Aang earthbending teacher so she would be the reincarnation of Roku's waterbending teacher and that leaves Zuko as the reincarnation of his earthbending teacher. Of course that means the Zuko must have died just before Tenzin was born, which is likely since it's been confirmed that he's dead.
 * Jossed  But destiny probably does has something to do with who teaches the Avatar.
 * Remember, though, Gyatso is the one who taught Roku airbending, and in his old age he taught Aang airbending as well. Katara thus becomes a Gyatso analogue.

An Avatar usually falls in love with a person from the next nation of the cycle.
Refer to theory above.

Aang falls in love with Katara. Air > Water

Roku falls in love with Ta Min. Fire > Air

Kyoshi... I don't know. She had... fans? Fan the flames? Maybe a tribute to a dead lover. Earth > Fire

Kuruk fell in love with Ummi, who had brown eyes, so maybe mixed. Water > Earth.

Yangchen gave a long speech that excused the Avatar from the Air quest to let all love and connection go. One can infer that she learned this from personal experience. Like the nun of Canterbury Tales, she is wearing an figurative pendant engraved with "Amor Vincit Omnia (Love Conquers all)." She also implies that she had to kill someone.
 * Ta Min was from the Fire Nation.
 * :cough: Hence the initial note "Refer to theory above."

The Avatar is A Time Lord
The reincarnation cycle is a crude form regeneration. And Katara is Aang's companion, just like Ta Min was Roku's.
 * Agreed. Waterbending and airbending together do not eternal youth in suspended animation make. He must have been messing with the wibbly wobbly timey wimey substance itself!
 * My personal version of this? Iroh's journey after Lu Ten's death involved a certain blue box and time-lord. If the Spirit World is just another dimension, it certainly explains a lot.
 * ... excuse me, I have a fanfiction to write.

Avatar Roku is the true mastermind behind everything.
Roku was in mortal danger when he faced the volcano, but he did not enter Avatar State. Why? For the same reason that, when his best friend proposed ideas he thought to be wrong, he immediately shouted them down with almost no explanation and stormed away: he was trying to start a war. This is also why his first action upon discovering the fire nation colony was to attack his old friend and threaten his life, despite the fact that an Avatar of all people should know that fire is a temperamental element prone to reactionary force (just look at any firebender). Roku knew about Sozin's comet (hence his conveniently timed death), and wanted to set things up so that the Fire Nation would take over the world, then the Avatar (still him, if he could 'awaken" his successor such that his portion of the soul was dominant - note how good his dragon is at putting ideas into Aang's head) could replace or simply ally with the Firelord, and bring the proper balance to the world and the elements using whatever methods he felt necessary.
 * If you put it this way, you could say that Roku's plan was a full success.
 * Ummmm why would he cause the death of so many just to go back to square 1/

Aang is the last Avatar
The reason he can't enter the Avatar State isn't because one of his Chakras was messed up, it's because Azula killed him under Ba Sing Se; fortunately, Katara had that vial of spirit water. As Roku explained, if he dies in the Avatar State, the cycle of reincarnation is broken, and the avatar is no more. Sure, Aang's still alive, but since he died, the whole avatar thing is now kaput. All the talk about restoring balance actually refers to showing Benders of all types that they can Bend any element they like; notice the foreshadowing of this idea when Iroh explains about the four nations to Zuko and how waterbending principles can be applied to firebending.
 * Rebuttal: While unconscious from Azula's lightning bolt, Aang undertook a journey to the spirit world to ensure the safety of both his soul and his previous incarnations, as was shown in the companion online game (script located here: http://www.avatarspiritmedia.net/transcripts.php scroll down to specials, and look for the labels "Avatar Escape online comic.) Secondly,  As to Aang being the ''last' Avatar...
 * Jossed by the new Avatar series coming out, The Legend of Korra, about the next avatar after Aang.

The Avatars knew that Energybending was possible, but didn't tell Aang about it because of the potential danger.
We know from 'The Library' that at least one Avatar has encountered a Lion Turtle before. We also know that in order to steal one's bending, one has to engage in a mental battle with the victim and win. If Ozai had succeeded, he could have taken one or more of Aang's bending abilities. Heck, he might have been able to take the Avatar Spirit! The result is an Omnicidal Maniac capable of bending all four elements, hyped up by Sozin's Comet and able to go into the Avatar State about to launch a campaign of genocide against the Earth Kingdom. With the obvious risks involved, the Avatars decided it would be best to tell Aang to kill Ozai, putting the stability of the world over Aang's morals in more ways then one.
 * Word of God is that there have been other Avatars who have mastered Energybending.
 * Only the last Air Nomad Avatar explicitly said that Aang should kill Ozai, the rest told him to mete out justice, be decisive, and pay attention (or something like that). Removing his bending ablities so he's pretty much harmless seems to fit. Though not mentioning spiritbending is a prudent move if they're afraid Ozai could destroy Aang's spirit, and making a second megalomaniac with all the Avatar's powers.
 * Possibly they were leaving it up to Aang to make a decision with his own moral compass, rather than making him consider something that goes against what he believes. (rather important for an Avatar) Whether it be killing him, depowering him, imprisoning him, etc.

Non-avatars can bend more than one element.
Everyone always takes it as fact that only the Avatar can bend more than one, but all that I recall being confirmed is that only the Avatar can bend all four. My guess is that it just takes a very long time for non-Avatars to learn, and they can't learn their opposite element (ie only an Avatar could bend both Air and Earth).
 * It could be possible for a person to learn to bend all four elements- it would just take a long time, and require an intense amount of training and concentration to do it.
 * If the experience of the Avatars is anything to go by, the complimentary element to the birth element is fairly easy to pick up, so a talented bender might be able to learn the element complementary to their first element relatively easily, but mastering both within a single human lifetime without drawing on the accumulated wisdom of multiple past lives would be challenging. The opposite element is very hard to learn, so perhaps an extremely talented non-avatar bender might then be able to master two elements and pick up the rudiments of the third, but mastering three would be nigh impossible. The fourth element is actually contradictory and deceptively difficult or downright dangerous, and even trying to learn it is probably a bad idea. Any Airbenders bold/crazy enough to try to learn Firebending, for example, probably got themselves (and any bystanders within range) killed early on.

Avatars rotate genders, and flip between Fire and Air.
Ok, I'm guessing the universe doesn't want to seem sexist, so it rotates the gender of the Avatars between male and female (Hence, prior to Aang the previous four Avatars were female-male-female-male). However, if they just kept flipping then each element would only get Avatars of one gender. This is why Aang, following Roku, was male. Last time Air and Earth each got a female Avatar, and Water and Fire got a male, this time it will be reversed (altneratively, Aang's a woman). This actually leads to an interesting implication: Since the Airbender is the start of the cycle, we can logically conclude that the first Avatar was an Airbender.
 * Unfortunately this is impossible. As we see in the first episode of series 2 the Fire Nation Avatar before Roku was also male.
 * I think that was just A previous Fire Nation Avatar, not necessarily the one immediately proceeding Ruko.
 * This could work, only it flips at every 5 incarnations of the Avatar to allow for the 'two male Fire Avatars in a row' problem. Aang is a flip point, said male Fire Avatar was a flip point, and 5 incarnations after Aang, the next Water bender Avatar after Korra will be male. This is why no-one in-universe has picked up on the flip- they expect it to happen at a set point, i.e. air, in the cycle and also, no-one has lived for more than 2 Avatars due to dying of old age.
 * According to the Avatar wiki, the Avatars are randomly gendered (in theory). More details here.

The Avatar cycle now has five steps.
Energybending actually being forced into Aang instead of being there as a passive thing mixed it up a little. Now instead of three nations and a group of nomads who vary between nomadding and templing, it will be three nations, a group of nomads, and a temple-dwelling race. The Nomads may be Aang's new people, mostly non-bending water travellers, and the temple-dwelling race may be students of Zuko the Balanced Lightningbender.

It's only the Avatar who can master all four elements.
Anyone with the right spiritual balance can learn all four elements, but it takes lifetimes of previous opposing-element Avatars within you to actually master your opposing element. Otherwise, a student of all four elements will only be able to master somewhere between zero and three and can only be passable or barely novice-level at the last one.

There's already a Water Nation avatar as of Season 3.
Katara's water brought him back to life, and Aang bringing the Avatar spirits and his own into a safe place in the spirit world did nothing to stop the Avatar line from continuing. The new Avatar won't be able to access the past Avatar spirits until Aang dies, but will be able to learn all four elements if anyone bothers to teach him or her how, or if the new Avatar just happens to start earthbending.
 * This troper is officially subscribed to this theory!
 * Aang died and so did the avatar spirit, if he died so the avatar would never happen ever again.

An Avatar could potentially read or manipulate minds.
Using a different form of Seismic Sense, only with firebending rather than earthbending, one could potentially sense the electrical signals of neurons. If one could figure out how these signals correlate, they could read someone’s mind. Waterbending could be used to sense hormones, and waterbending or lightning bender could be used to actually brainwash someone.

The Avatar cycle goes Male Female
But not through each incarnation, through each element. So Aang male Air, last Air Avatar female Yangchen. Korra female, last Water Avatar male Kuruk.
 * both Roku and the previous Fire avatar were male.

The First Avatar had to be of Virgin Birth
Think about it for a second. You have four elements entirely out of balance, primitive tribes using them to forge empires and massacre each other. Then a single woman bears forth a savior-a person who can use all four Elements and master them, and then starts bringing balance. The Sun Warriors, Ancient Air Nomads, Whatever Earth and Water Nations of the Ancient Paths are forged then. Yet no one can tell who the first Avatar's father is. It's a miracle that he appeared. Its because the Spirits chose this woman to bear the first in the Avatar Cycle. It wouldn't surprise me if the Avatar Mythology has their own version of the Virgin Mary.

The first Avatar was an airbender.
This was said in a previous guess that since the first element in the cycle is air, the very first Avatar is an Airbender. I'd also like to say that since Airbenders are the most spiritual of all the nations, it is possible that one of the Airbenders wanting to keep the world in balance, went to the Lion Turtle and the both of them found out a way to create a person from each nation to not only keep the balance and wisdom from the previous avatars, but bending the other elements as well.
 * But the first element in the cycle isn't air. Don't the elements correspond to seasons? Earth-Spring, Fire-Summer, Air-Fall and Water-Winter (At least, that's what I always thought). By that logic, the first Avatar would be an Earth Avatar.
 * If you go by the beginning of the Roman Calender, It would be a water one, as winter starts and ends the year. Same thing on the Chinese calender.
 * Actually, the first month in the Roman Calender is March. It wasn't until Christianity overturned the Roman Empire that the months were renumbered so that January was the first month instead of the eleventh, placing the Feast of the Circumcision of the Lord as New Year's Day.

The Avatar gets stronger with each generation.
Consider: Aang was an Airbending master at 12, and within a year he at least became proficient in the three other elements. Yet previous Avatars, no matter their prowess, were not informed until they were 16, and would take years to master the other elements. I don't think it was explictly stated but it at least appeared to me that Roku was younger when he mastered earth than Kuruk when he mastered air, and the shaved forehead probably confuses things but he looked younger still than Yang-Chen. If nothing else, each Avatar gains the strength and knowledge of all past Avatars when they enter the Avatar state, which would imply that each successive Avatar's power upon entering the state is greater than the last.

The avatar is a ghost
He exist as a form that will always be around, and he can communicate with the spirit world, and at times enter it. He know all ways of bending because he has literaly lived each and every style atleast 100 times. He forgets because his old memories merges with that of the host bdy he is possesing. When the host dies he exits and finds a new one on the next Elemental cycle, for ceremonial reasons. His original form was friends with that Guru in the episode where he learned to control his Avatar state.

Katara was supposed to be the next avatar after Aang.
This of course is after he died (if he had not frozen himself in the iceberg). Think about it why else would she be really good at waterbending if she wasn't supposed to become the next in line after him.

Being the Avatar is a Curse.
Like in many Eastern religious traditions, there is an afterlife beyond reincarnation. This Nirvana can be achieved by dying in a state of true enlightenment, and is a state of perpetual peace. All souls in the Avatar universe can reach this state after enough lifetimes, but the Avatar is cursed to remain, duty-bound to protect the nations until the end of time.
 * Guru Pathik tries to teach Aang to reach this enlightened state by unlocking all of his chakras. He says that once Aang does this, he will be able to control the Avatar state and enter into it at will. Had Aang truly mastered the final chakra, detaching himself from the world, he would have remained permanently in the Avatar state, eventually dying in a state of enlightenment. As Avatar Roku said, if Aang were to die in such a state, it would end the Avatar cycle — the Avatar would reach Nirvana.
 * Unfortunately, the Avatar never seems able or willing to fully unlock that last chakra. In almost every instance in which Aang enters the Avatar state, he is brought back by Katara — his last attachment to the mortal world. And the same sort of thing is implied to have happened to Avatar Kuruk — after a life of seeking detachment from the world's suffering, he is re-attached to the world when Koh steals the face of the woman he loves.
 * It seems likely that every Avatar, reflecting on the generations of suffering s/he has lived through, has had at least one moment of doubt wherein they contemplated remaining in the Avatar state. But each time, their attachment to the world has held them back, forced to stand on the precipice of paradise, but not entering until every other soul has been safely shepherded through. No wonder Aang didn't want to be the Avatar.
 * Buddhism knows a condition known as Bodhisattva, which means an enlightened person who consciously refuses to enter Nirvana in order to help others to achieve it. This, I feel, is pretty close to the condition of the Avatar in the series.

It takes an avatar an average of twelve years to master the elements.
Sozin said that Roku was gone 12 years. I was thinking that it'd take 3 years for them to learn the next element in the cycle because it comes easy to them (Just see how fast Aang learned waterbending!), then 5 years learning their opposite element, then 4 learning the last element. The last taking 4 years because it seems to be the easiest to gain but the hardest to master.
 * In the Avatar and the Firelord, Sozin said that Roku was gone for 12 years. Considering how Roku said it took him awhile to learn waterbending because it was challenging to him, I thought that maybe it'd take about 5 years to learn one's opposing element. 3 years is how long it would take to learn the next element in the cycle. (Look how fast Aang was able to master waterbending!) And then the last element would take 4 years, because its the easiest to pick up but the hardest to master.

Power taken by Avatars through energybending goes straight into the Gate of Truth.
If this seems like the "deception of the devil" idea long above linking energybending to the release of Homunculus Father from Fullmetal Alchemist, that's because this is a revisited version. After thinking about it for a long time, the "devil inside the gate has been working from outside this whole" idea doesn't make too much sense. However, the "energy bent by Avatars went into the gate" part, does. That energy still had to go somewhere, after all. So let's say it did lead to the release of Father from the Gate anyway, but not because a devil had any control over it. Rather, it was because, as the Avatar is essentially the spirit of the Earth in human form, the energy that was bent into the Gate is what allowed it to enable the harnessing of the energy of the Earth in a way smiliar to but separate from bending. In other words, not only was Aang the first ninja in thousands of years, his actions paved the way for the very birth of alchemy itself. This also meant, though, that bending dies off in the next 1500 years. It could be contended that The Legend of Korra josses this theory because Republic City is easily more technologically advanced than, say, Xerxes. But what if that level of sophisticated tech didn't reach much farther than the Republic (and Fire Nation) and then something else happened that eventually drove it all back? Flight vehicles, which there were plenty of in ATLA, don't seem to exist in FMA either, so even the earlier version of this theory was working on similar logic from the getgo.

Lion Turtles created the first Avatar.
They used Energybending to turn an Airbender into the first Avatar and jump-start the reincarnation cycle. They did this because they were sick of being in charge all the time or something. And when the world really doesn't need an Avatar anymore, the Lion Turtles will take the other three elements away from the last Avatar and thereby break the cycle.
 * Alternitively, the Avatar Spirit was a Lion Turtle, or a congolmeration of Lion Turtle spirits.
 * Alternatively again, the Lion Turtles used Energybending to fuse 4 benders, one from each element, into the Avatar Spirit. That way, the first Avatar, and all others thereafter, could bend all four elements.

Killing the Avatar in the Avatar state would destroy the world
It is said that if the Avatar dies in the Avatar state, the Avatar Cycle would be broken. This could kill the Avatar Spirit itself and the Avatar Spirit is the Spirit of the Planet. From what we know of what happend to the moon when the Moon Spirit died, the world would fade away if a replacement for the Avatar Spirit is not found quickly enough. Azula, you crazy bitch, you almost killed the whole world.
 * Which also means Katara saved all of existance as we know it for the Avatar world by not helping Zuko. I believe this now.

The first Avatar was 1/4 of each Nation.
His/Her mother was half Water Tribe/half Fire Nation, the father half Earth Kingdom/half Air Nomad.

The Avatar is descended from... the Avatar
Specifically each Avatar is descended from the previous Avatar born to that element. So Aang is descended from Yangchen and Korra is descended from Kuruk (there has been canonical emigration between the North and South Water Tribes). Nobody realises this because Avatar Kyoshi was around for so long that any records that might prove it are so old and obscure that no ones ever looked them up. Wan Shi Tong might know but he's not telling.
 * Made even better when you realize that all future air benders-including the next Air Nomad Avatar-is bound to be a descendant of Aang.