New/Modified Story!
Didn't Nomura say that all 358 days were originally planned to be playable? I would sincerely like to see that.
Maybe go into deeper detail of Saïx and Axel's relationship. And more about Xemnas+Xigbars connection/obsession with Ventus. And the final battle against Xion. Instead of her voice with Sora's body, mix their voices together. And after defeating her fourth armored form, another battle ensues. Xion's Ultimate/Final form is revealed to be Sora, although she not absorbed Roxas, Xion resembles Sora in every way (physical appearance, voice, personality and she literally believes that she is Sora.)
New/Expanded Locations!
How did Hollow Bastion miss this game? It's like the original home of Organization XIII! There is so much to explore there too. And it is about time that we got a playable Destiny Island, I say. And although it would be deserted and lackluster of story, I think it would be fun to revisit Traverse Town; it's probably swarming with Heartless.
New Weapons!
How did Zexion not get a giant DS/DSi for a joke weapon! And umbrellas? Why not big wind-up keys? Xemnas would look so funny with Chinese yo-yos or ribbons or something.. And cans of silly string or confetti cannons for Xigbar. Assorted sports equipment for Xaldin (baseball bat, golf club, hockey stick, croquette mallet, javelin, and s fishing pole). I guess Vexen could get like a police riot shield or maybe one of Goofey's shields? (Shields are so lame). Lexaeus could get a big ass boomerang hahaha. Like I said before, Zexion's already got a notebook, but where's his Nintendo DS/DSi?? Saïx totally needs like a giant thermometer and it could like rise in temperature when he get's closer to entering his Berserk Mode. As for Axel, how about some frisbees? Demyx's joke weapons are golden as is. A deck of Castle Oblivion cards for Luxord would be total fan-service to teh max.. Or a deck of Kingdom Hearts brand trading cards hahahaha. Marluxia's is hard, what about an old farming scythe? A set of forks and spoons for Larxene. Or some suction cup darts lol. And for Xion.. You know those dolls that are like a head+torso and then the rest is like a stick? She should get one of Sora! That or a Kingdom Key that looks like it is made of card board or something. And of course some new 'serious' weapons.
New Game +!
After completing Roxas' story, Xion and Castle Oblivion's story is unlocked. Xion's would be a recap of 358/2 Days through her pov and Castle Oblivion's would cover when Naminé is born, the Organization's discovery and occupation of the Castle, and their demise from their pov.
New Game ++!
After completing New Game+, DiZ/Riku/Mickey and Remnants of Organixation XIII mode is unlocked. DiZ/Riku/Mickey would cover their motives and actions starting from the end of CoM leading to the end of KH2 and Remnants of Organization XIII mode would recap their motives and actions leading from the start of KH2 to the very end of the Organization itself.
Additions to Multiplayer Mode!
I love the Struggle, make that multiplayer! Add a

recreational/arcade room to the Castle that Never Was for fun Organization multiplayer minigames!:
- Xemnas Flag-Waving!
- Xigbar Can Shooting!
- Xaldin Pole Vault!
- Vexen Snowboarding!
- Lexaeus Whack-A-Mole!
- Zexion I Spy!
- Saïx Moon Phase Memory!
- Axel Burn-Out racing!
- Demyx Says: Musical Notes!
- Luxord Casino!
- Marluxia Landscaping!
- Larxene Darts!
- Roxas the Heart Collector!
- Xion Doll Dress-Up!
Should I even mention Wi-Fi?