I love X-2, one game I actually enjoy fighting to gain abilities. Plus Lenne and Shuyin are so cute, and Paine, and Baralai! X-2 needs more love.
Maybe I'm just nitpicking here, but does it bother anyone else that the only Final Fantasy characters to appear in Kingdom Hearts are from 7 and up? With the exception of Setzer, who was butchered pretty badly. Nomura has proven that he can redesign Amano's characters pretty faithfully with Dissidia, and I'd love to see Terra or Cecil in KH someday.
I just want more IX characters in it to be honest. Zidane could fit right in, Dagger and Steiner in a castle setting perhaps, they wouldn't need much re-designing either. Anyone from the pre-VII games would be lovely though - ah, I can imagine Faris and Bartz in it, that'd be heaven for me, hah. As long as they don't add any more VII characters, we have more than enough, thank you Square.