I'd love it if they somehow brought Balthier and Fran in, they'll work so well off Sora's personality.
But forget the final fantasy characters, which PIXAR world do you want to see? It's just about the right time for them to make an appearance, what with this new generation of tech to do the Pixar CG justice.
Incredibles would be a perfect fit and Toy Story would be the safe bet, but personally, I'd be squealing like a little girl if they somehow manage to smash in WAll-E.