Would definitely like to see more female fighters thrown into the mix.. What they did to Terra was embarrasing, but at least they included her over Locke.
Rydia would be a great choice for IV, but if they keep the same summoning system I'm not entirely sure how she'd work.. her whole thing was being a summoner. But I do think having a certain class of character that could do that would be pretty fun. I'd also quite like to see Kain and Rosa.
Farris from V, Celes from VI, Tifa from VII, Laguna from VIII. Dagger and Yuna would again work as summoners, otherwise they could use the gun wielding Yuna from X-2 and Stiener from IX, who is easily one of the best characters from the whole series. I also very much liked Beatrix, but I'm not sure how likely it is they'd include someone who was only a temporary party member.
Ashe or Bashe from XII and Lightning from XIII. And I sincerely hope they keep anything from Kingdom Hearts very far away.