I'm not sure if I want a sequel, even though I love the game and the characters and the world, but still... it could be ffx-2 all over again :/
But if any of those, then it would be Laguna's story in-depth mixed with the Sorceress War (well it was about the same time so it'd make sense). Also I'd probably like to see a glimpse of the heroes of the original game as children. :P