I'm not a big X fan, but I think Spira is an awesome world and both games have great gameplay.

Color me excited. I think an open-world RPG in which you can choose a race (Human, Al Bhed, Ronso, Guado, Hypello), a class (Fighter, Thief, Monk, White, Black Mage etc.) a la Elder Scrlls would be awesome. Add some New Vegas-esque political intrigue and my money would be yours.