That would be awesome--he is my most wanted character for the game, and he would fit perfectly. Here are my ideas for his moves:
(with GC controller in mind)
A button: fast, weak keyblade attacks
A+Direction: Power keyblade attacks
B button: light Fire attack. Similar to Mario's fireballs, but about 50% stronger, with 1-2 frames more paralysis time. Can fire 3 times before having to stop for 2 seconds.
B+ left/right: Holding less than 2 seconds= Fira attack. Firaga for more than 2 seconds.
Up+B: The Air Combo move where he jumps forward into the air and swipes the keyblade upward.
Down+B: one of two options-- 1. a Cure spell that heals 2% but takes 80-100 frames, and you only heal at the end of the frames.
Final Smash: Trinity Limit, naturally!
Other stuff:
Roll would naturally be a dodge roll animation, shield would be a Leaf spell
OPTIONAL: Could have a glide ability for recovery.
Also, incidentally, I had a dream last night that KH3 was announced for Wii. It's graphics were halfway between PS3 and what we've seen from the best Wii games.