Grand Unifying Guesses/Avatar: The Last Airbender

Identified reincarnations of the Avatar

 * Yangchen (duh)
 * Kuruk (duh)
 * Kyoshi (duh)
 * Roku (duh)
 * Aang (duh)
 * Korra (duh)
 * Luke Skywalker
 * And Anakin before him?
 * Ash Ketchum
 * Naruto Uzumaki whom, if the magical-ninja-element-litmus-test is to be believed, would be the first Airbender after Aang.
 * The Doctor
 * And it logically follows that the Avatar is a Time Lord. He/she just has a slightly different method of regeneration.
 * And Katara is his companion (First row, third panel)
 * To be precise The Doctor (formerly known as Iroh) was the avatar immediately before Yangchen.
 * The Singer of All Songs, in the book of the same name, though the specific Singer Of All Songs was not actually named in that book and regarded as improbable myth, so this incarnation may have been harder to identify, or may have gone Beyond the Impossible and outright split into a possible GROUP
 * Merlin
 * Neku Sakuraba
 * Minato Arisato
 * Abraham Lincoln
 * And, by extension through the Time Lord theory, all U.S. Presidents.
 * Steve Wozinak
 * Yugi Moto/Yami (The Avatar of Card Games)
 * Jaden Yuki (The Second Avatar of Card Games)
 * Yusei Fudo (The Third Avatar of Card Games)
 * Harry Potter
 * Ellis
 * Renee Tempete (I Hate Gallant Girl)
 * Captain Planet
 * Omi from Xiaolin Showdown. He even mastered all four elements and entered an erstaz Avatar State in one episode!
 * WALL-E, which SHOULD prove that the Avatar is able inhabit a robotic state
 * Near
 * Superman
 * Sankt Kaiser Olivier. She's worshipped due to the fact she broght a very large war to a peaceful conclusion, if that isn't enough to show that someone's the Avatar nothing is.
 * Storm from X-Men complete with Glowing Eyes of Doom
 * Altaïr (He Assassinated all the other ones.)
 * Simon
 * Lord Genome
 * Terra
 * Donnie Darko
 * Twilight Sparkle (note how her eyes glow in that one flashback)
 * Donnie Darko
 * Twilight Sparkle (note how her eyes glow in that one flashback)

The Avatar is related to The Avatar
Sometime after the events depicted in Oban Star Racers, Avatar Jordan realized that a single being having that much power was a bad idea, and decided to divide the powers so each planet would have its own (much weaker) avatar. He also made it so that when an avatar died his/her avatar spirit would transfer to someone else as opposed to determining the next avatar via a race.
 * Alternatively, it's the other way around after several generations of avatars, one was powerful enough to maintain balance throughout the universe galaxy, be for most intents and purposes immortal, and could even alter the nature of the avatar cycle. He/She feared that the next avatar would become corrupted by the avatar's power and created the Great Race of Oban to decide on a successor rather then relying fate/luck/the-avatar-spirit's-judgment. (Presumably, people who were likely to become corrupted would try to kill their opponents during the race if they thought they could get away with it. Furthermore, those same people would be more likely to be sabotaged by angered teammates).
 * Given the fact that the Avatar is unable to leave Oban, and the presence of monkey/lions (called "Drudgers") we can conclude that Oban is the planet that Avatar: The Last Airbender takes place on.
 * Ah, but are either related to The Avatar? Or even... THE OTHER ONE! Avatar, the one on the moon Pandora, of Jupiter (Well, the big storm LOOKS about the size of 3 Earths.)