lets talk about it:
-Immensely popular character not yet already in a fighting game (more hype+sales)
-Potential for incredibly diverse moveset
-Actually fits well in a superhero theme'd game/setting
-Uses environment to fight as well as many gear options
His moveset should have some good 50/50s with emphasis on aerial attacks with okay but punishable anti air moves. His normals should be slightly on the slow side but he should have a fast run speed, decent recovery etc. His moves should incorperate some elemental moves meaning some small projectiles going with it. He should make use of his staff too.
He should have a counter/parry move since one of his MA skills is to "wait and listen".
For example, D2 should have him sweep the opponents feet with an air blast, which would be comboable but very easily punishable. However it should have decent range as well.
His trait would be avatar state. it would get 2 levels, at level 1, he would use air to push himself to avoid projectiles, moves etc while greating distance. , level 2 would be a great air attack that is mid screen where aang's eye would glow for a moment while doing the attack, the opponent would get sent flying up in the air (comboable but hard to do). This would enhance Aang's next few moves or so.
his supermove would be over the top, it would begin with him using air to push the opponent down (if they dont block it that is) then while on the ground, usiing the avatar state aang flies up into the air using the earth to launch them into the air where a wave of water would slam down on them. While face down aang surrounds them with fire making alot of steam before finally wacking them with his staff.
aang is the "superman" of his own world, and a pacifist who actually would greatly oppose killing even when its for the greater good. this would fit well against an oppressive dictator...even going as far as stopping himself from killing which had led him to be vulnerable at points, but ultimately fights for peace.
Control over elements, using real martial arts to use them. enhanced speed, strength,etc.
Uses a staff to enhance his abilities.