Well Final Fantasy XI has been around for awhile, and I don't know if Square-Enix plans to end it anytime soon (they do have plenty of players) but if they do decide to make a successor to Final Fantasy XI, do you think Square-Enix actually could pull off an Ivalice MMO?
I mostly was thinking about it from the adventures of Clan PissBagel in Final Fantasy Tactics A2 and plyaing Lion war about a year and a half ago. (or however long it's been since it was out. *shrugs*) I think it actually could work. They got a pretty expansive world set up for Ivalice already, although they'd have to make some bigger areas.
I don't remember what organization Kuja's half-sister Ilua was in but Cid #3 actually hinted that they didn't really destroy it so it could sort of be a background antagonist or just one of the antagonists with story arcs behind them they could always just invent others. (Like WoW did, cept Ivalice is more of a work in progress compared to that.)
they also have some side-characters like Vaan and Penelo since there's always treasure to steal. Same with Fran and Balthier, Hurdy, Cid #3, Al-Cid the womanizer (who flirts with you if you have a female character) montblanc and Colette Bru-err I mean Adelle. They can have "Guests" where they help for a boss encounter or something.
One important thing though would be class balance. Would you be able to just play a class like Ninja right off the bat or would you have to play a thief to progress? If you had a "Progression" system then it quickly would just be a bunch of high-tier classes like Parivir and spellblade running around and some powerful class like ninja or Sage with relatively easy requirements would probably have hundreds of players simply because stuff like Assassin would take awhile to unlock.
Though what about how Final Fantasy XI allowed you to change classes without having to create a new character? How would it work in an Ivalice MMO?
And yeah, Race balance can also be mentioned but they can easily do that...they all move the same speed, Gria can't really fly, and their classes would suggest stuff like a melee for Bangaa and caster for Nu-Mou.