Oh, man, I've been waiting for a thread like this:
1) Assign guarding to a shoulder button. I died so many times in KH because I could never get the timing down for blocking. I think the best approach would be the God of War 2 style of guarding, in that holding down the button blocks most attacks, but tapping it the right before the attack hits would store the energy of the attack, or reflect the projectiles. Yeah, that could work...
2) Make two shortcut menus. One could be used to assign Magic and Items, and they would be selected with the face buttons. The other one could be for selecting Drives, limits, and summons, and could be mapped to the directional pad. Just hold down L1, and select the thing you want to use.
3) Give us more freedom in customizing Sora and the party members. Preferably, allow us to distribute stat points whenever a character levels up. I think the Torchlight approach would work best, where you selected a "class" (in this case, sword, shield, or staff) at the beginning, but are free to customize the characters however you want. Each time you level up, you get 5 skill points to increase your Strength, Magic, and Defense, and you get to select a new ability from a "tree" of abilities. This would allow us to create our own party, and add a bit more depth. If accessibility is what you're worried about, then just put in an auto-leveling option for new players.
4)Worlds from other Disney movies. I'd die to see The Hunchback of Notre Dame in KH3, and Pocahontas and other older Disney worlds would be a treat too. Heck, even put stuff in from the recent GOOD stuff (Atlantis: the Lost Empire, Treasure Planet, Dinosaur, etc.), or, if you can, I'd like to see some stuff from PIXAR (Toy Story in KH3, anyone?) And hey, since Disney owns Marvel now, let's see some Marvel characters! The list goes on! Just PLEASE no Disney Channel garbage, please?
And if I see ONE reference to Jonas Brothers, High School Musical, or Hannah Montana, I'm burning my disk.
5) This might be a bit of an overkill, but how about seeing Sora use some of the Organization's weapons? Maybe he has to defeat a ghost of an Organization member, then when he kills it, he gets their weapon! Xigbar's guns for long range, Lexaeus's tomahawk for heavy damage, Vexen's shield for Defensive, or maybe Demyx's Sitar for summoning expendable goons! Be creative!
6) Have Sora and Kairi kiss. I don't think I need to elaborate with this one.
Well, sorry for the epic length post, but I've had these ideas floating around in my head for a while now.
SE, if you're reading this, and are wondering what to put in there, then please, for the love of God, INCLUDE #6. Everything else is mere gravy in my eyes.