Well considering FFX-2 bankrupted them (along with FF: Spirits Within) I say don't make a sequel.

Final Fantasy X-2 didn't bankrupt Square.  The movie did that and solely the movie.  Hell Final Fantasy X was possibly the last Final Fantasy game.  I don't think it was even gonna be finished if Sony didn't give them the money for it; in exchange for 20% of the company's stock (Though I'm not sure when Sony took the stock, if it was before or after Final Fantasy X).  Though I'd personally like a sequel to Final Fantasy XIII, all the backstory that wasn't in the game, or was hidden away in the datalogs and the only way you can see them is if you get all the missions done... maybe they can tell now in a sequel.  I mean even though the story in Final Fantasy X-2 was a joke, the one thing it did right was take its time and let you enjoy the world and side stories.  That was severly lacking in Final Fantasy XIII and making a sequel with that added in I'll buy and enjoy.