FFIV: Cecil, Edge
FFV: Ex-Death, Butz
FFVI: Lokke
FFVII: Cait Sith, Red XIII, Barret, Vincent, Reno, Rufus, Hojo, Zack, WEAPON
FFT: Ramza
FFVIII: Irvine, Edea
FFIX: Freya, Quina, Vivi, Zidane, Amarant, Steiner, Garnet, Eiko, Zorn + Thorn
FFX: Nobody. I don't particularly like this game, so I don't really like the characters. The only one I can think of that I would want in KH2 is Rikku, as Wakka is already in KH1, And there are screens of Auron in KH2.
I would also like to see Moomba's and Chocobo's in KH2, just to make it look cooler, and maybe even a couple of worlds which are FF based, instead of them all being Disney worlds.