Suikoden/WMG

Isabel from 5 is the mother of Fred Maxmillian from 3, and the daughter of Max Maxmillian from 1 and 2
Isabel is known as a member of the Maxmillian Knights, and looks quite young by the time of 5. She also said that her father is a knight. It is very possible that after the fifth game she got married with someone else (maybe with Mathias?) and lay down her sword to raise a family, forcing Max to once again take the lead on the Maxmillian Knights as she tries to hide the fact that she was also a Knight from her son Fred, which explains why Fred thinks that he's following the steps of his grandfather, not father or ''mother. The time gap between 5 and 3 should also be enough for Isabel to give birth to Fred, and for Fred to mature up to the guy he is in 3.
 * Its certainly plausible, but I thought Maximilian didn't retire, his unit was just disbanded after the war. Isabel's father did retire though, as it was with his sword/lance and shield she fought. Being an Expy of Don Quixote, I always figured Max kept fighting as a Knight-Errant, and never personally hung up his sword. Though if Isabel is his daughter, that'd be cool, as it adds yet another family to the recurring family list, as I believe Gary from 4 was confirmed to be an ancestor of Maximilian.

Rahal from Suikoden V is actually a biological female
So, the Dragon Cavalry doesn't accept women. Rahal is the spitting image of his twin sister when he cross-dresses (which he's stated to enjoy doing). After it's all over, Rahal reforms the Cavalry so that women are allowed into their ranks. Add it all up, and you get: Rahal wanted to join the Cavalry from a young age, but realized that "his" gender would be an obstacle. Instead of giving up, he begins dressing like a boy, but still finds wearing women's clothing to be more "natural."
 * Now part of my personal canon, just sayin'.
 * I take that as a compliment; given how much enjoyment the series places on figuring out the minutiae of your characters' lives for yourself, I was proud of myself for even coming up with the theory. The longer I thought about it, the more sense in made.
 * Quick question, what happens in bath scenes when you take Rahal with you?
 * A very good question. I may have to dust the ol' game off just to find out. (If Rahal appears with any sort of visible wrapping around the upper body, I just might take that as proof.)

Leknaat is Viki after her adventures throughout the series
Viki's teleportation abilities are all about sending stuff from where she is to elsewhere, which seems to be the same thing Leknaat does with her half of the Gate Rune, and there's precedent for runes being misidentified as a weaker version of themselves in earlier games, most obviously Luc's True Wind Rune in the first two games. Thanks to Young!Viki in 3 we know she can exist without the whole Dojikko thing, she's a confirmed Time Traveler, and it's easy to see the future when you've been there before.
 * This also came to my mind based on her appearance and her abilities. But yes, I find this an interesting theory nonetheless. I often also thought that she was Viki from another dimension all together.
 * Well, lets see first of all Gate rune could not be used for time travelling. Second (this fact maybe unknown to people never played Suikogaiden) in Suikogaiden 2 we can see Viki's homeland. And as far as I can say It looks nothing like Leknaat's homeland.

Suikoden V's Roy is distantly related to the Prince... by virtue of being a bastard of one of the Barows.
In the past, members of the House of Barows have married into the royal family -- Arshtat's own mother was even married to Salum Barows' cousin! Note that he also grew up on the streets of Rainwall, which just so happens to be home to the House of Barows... This would help explain why he resembles the prince so closely that even Oboro investigates if it's actually an Uncanny Family Resemblance -- he can't find any evidence directly connecting him to some member of the royal family being unfaithful, but that doesn't mean that some relative of the Barows shared such high moral values...

Nash isn't married.
He just made up his wife to make Chris feel more comfortable when he's getting too flirty and too keep her knights from freaking out too much over turning a blind eye when she sneaks off with a strange man; they don't need to worry, he won't really do anything beyond flirting, he's a henpecked husband! I was honestly shocked when there wasn't a reveal of this after my first time through the game, since to me it seemed obvious that the scene where the knights watch them leave and Louis guesses just that was the child seeing the truth that flew over the heads of the adults.
 * Wasn't it in Suikogaiden where they reveal who Nash's wife is? Because Suikogaiden was all about Nash's story after all.
 * No, Nash isn't married or in a relationship with anyone in those games. People guess his wife is Sierra from them because they get along well during the (comparatively) brief amount of time they spend together in the first one, because when he describes his supposed wife she and Sierra seem similar, and possibly in part because he doesn't spend that much time with other women who aren't related to him or already in love with someone else during the gaiden games.

The True Runes are based on the Tarot.
Take a look at how many True Runes there are. 27, right? Five of them are associated with the elements, leaving 22. It so happens that there are 22 major arcana in the Tarot. As for the five True Elemental Runes, they are analogous to the Minor Arcana as a whole, each of whose suits was associated with one of the Greek elements (we have five here because of the connection with Chinese elements). Some guesses on which True Runes are which Major Arcana follow:
 * The Fool: Rune of the Beginning (may instead be the High Priestess, though...)
 * The Magician: Dragon Rune (association with direct, active strength)
 * The Emperor: Sovereignty Rune
 * The Hierophant: Circle Rune (the Hierophant represents tradition and the authority of the past, redolent of the Circle Rune's stasis qualities)
 * The Wheel of Fortune: Change Rune
 * Justice: Moon Rune (the rune is supposed to exemplify both cruelty and compassion. The administration of justice, both to succor the wounded and innocent and to bring down the wicked, is an obvious meeting point.)
 * Death: Rune of Life and Death (even if Death usually signifies a point of no return, rather than only actual death)
 * The Devil: Beast Rune
 * The Tower: Another candidate for the Rune of Life and Death, fitting more closely with death as the arcanum of destruction.
 * The Moon: Night Rune
 * The Sun: Sun Rune
 * Judgement: Punishment Rune

Cougar of Suikoden Tierkreis was the Star of Heavenly Guidance of his world.
Most visions -aside from Diadora's staff and Rahrohenga- of the False Chronicles depicts, in some way, the leader, marked by being garbed in blue and white, confronting the One King, or as they are about to confront him. In addition, only the Tenkai Stars of other worlds, Atrie and Other-Marica, have appeared to the hero, and were also present, in their own worlds, during the final battle.

A future Suikoden game will play through The Hero's War and the founding of Holy Harmonia. as Hikusaak.
The series's continuity jumps all over the place, as does it's geographic focus, but two major "themes" shoot through a lot of it: Harmonia's interests and the True Runes, and what the former has to do with the latter. This fact that no matter who The Empire of the day is, Harmonia tends to be the Bigger Bad. And on a deeper level, there are fairly few parts of the backstory that don't tie back tot hem in some way. The very *calender* most people in the 'verse- and most games in the series- use is *centered* on Hikusaak and his asencion to power, and the fact that the Scarlet Moon Empire and Highland were once Harmonian indicates that most of the mainland might well have been controlled by the Empire at some point, meaning that the most influential countries on the continent have at least some Harmonian descent.

And so if the series does continue, it'd make sense for them to sooner or later come back to it, particularly considering that what little we know (or that Harmonian history *tells* us) of Hikusaak and the origin of the Empire bear *surprising* parallels with your average Suikoden game: lone hero sets out against dominant power of his day and unseats it, remaking the world in their imagine (if only in part, with I in Toran, 2 in Dunan, III in the Grasslands between the Tribes, Zexen, and Harmonia, IV in the South Seas, V in Falena and New Armes, to say nothing of the spinoffs...).

So, you know what this means? Dark Reprise ! Only this time, with extra *irony*. Particularly since we'd see how Hikusaak went from a Protagonist (even if a Villain one) to the Bigger Bad, La Résistance becoming The Empire, and why things work the way they do up North. Particularly since the traditional Suikoden/Water Margin system and the 72 stars would raise the question of why- if he could only succeed through the help of others (considering it's a pretty big aesop the series has)- Harmonia only recognizes *him* and why he took up the banner of the "One True Hero" out of all the 72 stars.

Of course, the game devs will try to conceal the truth of who you're playing and what by holding the reveal of the character as Hikusaak and his movement as the nascent Harmonian Empire in some way, intentionally contradicting what (little) we know about the events and justifying the handwaving away as being Written by the Winners and having their requisite biases, propaganda, and falsehoods about an event they view as all but biblical coupled with the rot of the years distorting what history of the time there is.


 * I take it to mean that this idea is for Suikoden's final game. Two popular events for the fans are the Scarlet Moon Empire Succession Wars (with Barbarossa as The Hero) and the First Fire Bringer War (with the first Flame Champion as The Hero). I could only hope they could tackle any of these first, but of course, I'm expecting the devs to end the series in Harmonia with a bang.