As much as I hate to say it, FFX would be the only FF to consider having a sequel to; unfortunately, Square Enix is really screwing it up out of what I've seen on FFX-2. It makes me wanna smack someone at Square Enix across the face with a dead chicken.

As far as prequels are concerned, FFX, FFVII, and FFVI are 'bout the only ones that'd have good prequels, since FFVIII's characters would only be younger than they already are, and not much happened before the beginning of FFIX. Perhaps if Square were to make a movie to be a prequel for FFX, that'd work best.