Sora, Riku, Mickey, Aqua, Ventus, Kairi, LEA.
Maybe Terra if they liberate him but it is still unclear if they will be able to save him or not. May be some heroic sacrifice for him.
Ideally, I would love some open world like thing where you can swap between characters at a time/in party. This would work after they have been saved in the case of Aqua and Ventus.
I would love them each to have different styles and level up stats and bonuses. I know Sora will be the main, but since there is 7/8 protectors of light I would like to use them all. Even playing on Kairi and Lea's lack of exp would be fun to do.
I also think that missions for each of them would be cool too. That would work for like a hub world like Mysterious Tower where they all meet anyway. Similar to 358/2 in style for these missions but they are only a part of the whole open worlds game.
DLC possibilities or even post game unlockables depending on what happens in the storycould be Xion, Roxas, Goofy and Donald.
Having played as Sora, Riku, Aqua, Terra, Ventus, Mickey, Roxas and even Donald and Goofy in 358/2 I think that it would be reasonable to let us play them in one form or another.