Personally, I want to see a new world, maybe some new Final Fantasy characters, but most importantly, I want to see another playable character. While I know that it's unlikely, I'd love if KH 3 had multiple parties you can switch in-between during the story. I'd like Riku and Aqua to be playable again, but I won't mind much if it's only sora, even though I will be a bit disappointed.
I definitely want to see Aqua return since she's the only female protagonist in the game that I actually liked. I just want to see more plot related stuff and how it will connect to the other two games.
I was thinking that even if Aqua won't be playable, perhaps Kairy will and she may have simmiliar gameplay to that of Aqua. I'd still prefer to play as Aqua herself, but any diversity is good.
Well, they kept hyping Kairi and even let her fight a bit with a Keyblade, so I don't see why she wouldn't play a bigger role in the game. Unless she gets kidnapped again and Sora has to go rescue her.