My picks...

Firion - Leon/Dark Knight
Onion Knight - Xande
Dark Knight Cecil - Kain (though I don't mind Golbez)
Faris - Gilgamesh
Terra - Kefka
Replace Cloud and Sephiroth with Vincent and Reno
Squall - Seifer ( other option is Laguna - Edea)
Replace Zidane with Vivi but keep Kuja
Auron - Jecht (human form)
Basch - Judge Master Gabranth
Paine - The head of the Machina Guild... can't remember his name cause I've blighted out this game from my memory...)
Ramza - Delita (seriously, anyone from Tactics would make me happy)
Marche - Ritz
Chocobo - Moogle