Hm.. let's see.. I can make the whole list, starting from FFVI:
FFVI
Prequel: The Magi War sounds really interesting enough. 1. As a movie 2. As a game
FFVI: Since I'm a pretty big fan, I would like to see a movie version of it, since there's nothing wrong with its storyline.
Sequel: There's not really something that comes to mind that would be good as a movie or a game.
FFVII
There's already enough. I'm glad that the FFVII Compilation is over. Let the bigger fans dream on on the remake.
FFVIII
Prequel: 1. As a movie (there's enough material, even though as good as all matters have been explained in the actual game, to make a decent movie, like the Sorceress War/Adel situations) 2. As a game, perhaps, but much rather as a movie.
FFVIII: It's not one of my favourite Final Fantasies, but the storyline is decent enough to make an actual movie.
Sequel: Same as with FFVI. ^
FFIX
I would love to see all posibilities for this game. ^_________^
FFX
Prequel: 1. As a movie, why not =) 2. Again, I'd rather see it as a movie.
FFX: Wouldn't really be in need a movie version, since FFX's graphics makes the whole of the game already as a FF movie.
Sequel. No.
