Huge fan of FF Theatrhythm, and I've noticed when I downloaded the extra content songs, that there are even songs from unreleased FF titles (Final Fantasy VS XIII, Final Fantasy Type Zero is coming up for download content, ect). One song I particularly love from XIV is Nail of the Heavens.

http://www.youtube.com/watch?v=_VM2cAZJjTA

If you could put this in Theatrhythm, think a lot of the players that also play XIV would love it, I know I would. Could even influence (real stretch) some people to check out XIV as well