Umm lessee...
Mars Volta doing Dancing Mad since it's a fairly progressive tune
Doves always have awesome instrumentals and are very eclectic making them a perfect group to do FF covers
Radiohead would be cool, but I think they'd bastardize it
SOAD would be hilarious, and hey they did the Zelda song
Flaming Lips would also be funny
HMM I think ENYA should do lessee FF4
OMG Yes should totally do it, they used orchestra often enough, once again progressive rock always makes things interesting, and the lead singer can do all the high-pitched girly opera parts jk