V on the PSP: All they'd need to do is continue to improve the dialogue and give more character growth.

VI on the PS3: they wouldn't really have to change much on this one, it'd be a killer game.

VII on PS3: this one would definitely be the most difficult to pull off with integrity. There were so many things in FFVII that were easy to do with deformed characters but are totally unrealistic with proportional models.
Jumping on a tower while riding a dolphin anyone? yeah.