Disclaimer: I in no way condone sequels or prequels to FF games.

I: A game set 1000 years in the past, and ties in with the time loop of the game.

III: Back when the Warriors of Darkness had to stop the overflow of light into their world.

IV: Events leading up to Cecil & Golbez's father coming to the world of FFIV

V: War against Euno

VI: War of the Magi

VII: Jenova vs. Ancients

VIII: Something set in the far future, Ultemecia's time.

IX: Whatever crisis befell Terra

X: Zanarkand vs. Bevelle. You could side with Zanarkand and in the end be the hero who creates Sin to keep your society from being wiped from existence by the evil people of Spira.