Hmm, I thought about this potential. Say Sakurai includes Sora in Brawl. Then I think the moveset should look something like this (it's all tentative and just personal opinion):
Neutral A - a regular Keyblade slash, with additional slashes for a combo. [3-4% her hack]
Forward Tilt A - Slapshot (KHII ability): what it says; Sora slaps the enemy multiple times for decent damage with little knockback. [4-5% per hack]
Up Tilt A - another simple slash, but with an upward movement; little knockback but good for setting up combos. [9-11%]
Down Tilt A - Upper Slash (KHII ability): knocks the enemy up, setting up combo possibilities. [6-9%]
Dash A - Slide Dash (KHII ability): just like his KHII ability, closes in on his target and whacks him/her/it with the Keyblade; has a slight lag, kind of like Marth/Roy's dash attack in Melee. [9-11%]
Foward Smash - Blitz (KHI ability): a powerful hack with good damage and good knockback. [20-25%]
Up Smash - Ragnarok (KHI ability): not quite as powerful as in the game, but a series of upward slashes much like Link's up smash. [6-8% per hack; potentially up to 24-28% when fully charged]
Down Smash - Vortex (KHI ability): a lagged attack that has high priority, decent range, good damage and knockback and leaves Sora slightly impervious to opposing knockbacks. [16-20%]
Neutral B - A random magic spell (Fire, Blizzard, Thunder, Reflect ). [7-10%]
Forward B - Strike Raid (KHI ability): throws his Keyblade with decent range; similar to Link's boomerang but slower, more powerful and limited homing ability. [11-15%]
Up B - Aerial Dodge (KHII ability): gives Sora a good vertical recovery but hard to sweet spot. [0%]
Down B - Explosion (KHII ability): a mid-range attack with decent damage but subpar knockback; a juggling attack with multiple magic balls around Sora. [2-3% per hit; potentially up to 18-22%]
Neutral Air - Aerial Spiral (KHII ability): flips in midair while exposing his Keyblade. [9-12%]
Forward Air - a forward-moving slash, much like Sora's aerial neutral attack in KHII -- a slash in front of him. [10-12%]
Back Air - A simple aerial roundhouse kick with decent knockback and little lag. [8-11%]
Up Air - Aerial Sweep (KHII ability): has some upward movement with little lag; good damage and potential aerial finisher. [12-15%]
Grab - just a simple grab with his other hand. [0%]
Grab + Attack - bash the grabbed enemy with the handle of the Keyblade. [2-3%]
Forward Throw - releases the enemy and whacks it with the Keyblade. [6-8%]
Back Throw - similar to Marth/Roy's back throw in Melee. [4-6%]
Up Throw - similar to the forward throw but upwards. [6-8%]
Down Throw - Sora leaps and slams the enemy towards the ground. What I had in mind was one of his Reaction commands against one of the Nobodies called Fail-Safe. [6-8%]
Taunt 1 - Cloud's victory fanfare
Taunt 2 - Tidus's victory fanfare
Taunt 3 - Squall's victory fanfare
Final Smash - Trinity Limit: a little mix of Sonic and Link's Final Smashes, dealing out massive damage with very good knockback and Sora's speed increases.
Costumes:
Default - Kingdom Hearts II (mostly black) - Kingdom Key
Kingdom Hearts I (White top, red bottom) - Oathkeeper
Valor Form (all red) - Sleeping Lion
Wisdom Form (all blue) - Fenrir
Master Form (all yellow) - Oblivion
Final Form (all white) - Ultima Weapon
Of course, the Keyblade change doesn't mean more or less range, power or knockback.
---
Yes, I've spent a lot of time on this, and yet, Sora may not be confirmed to be even in the game. Any changes or criticisms welcome.