I gotta agree, prequels would be more appropriate, and that of the Dawn Warriors would be on the top of my list. After you beat all the games, your characters are all the strongest warriors who ever lived, anything that could possibly rise up would be completely crushed, I thought "Advent Children" was good but that could never be put into a video game.
I think an awesome game would be for Sakaguchi to make the story of Garland, starting as a Knight of Cornelia, then what happened to make him break off. Then it can show what happened to him after the Light Warriors killed him, the process of becoming Chaos 2,000 years in the past and sending the 4 Elemental Fiends into the future.
AND THEN IT SHOWS WHAT HE DID AFTER FFI LEADING UP TO THE EVENTS OF FFIX!!!!