Okay, first off, my apologies if this has already been proposed - I did take a look around, but I didn't see anything related to the idea I had.
Second, the idea:
While the soundtrack for the game is very good, there are people out there who might not like it (or who might want to use their favorite music for the ingame soundtrack just as a change of pace). Thus, I was thinking that perhaps it might be possible to create a system for letting the players change the soundtrack of the game by themselves.
I was thinking of something fairly simple - a section of the Options menu (possibly a subsection of the Sound section) where you could set up a custom ingame soundtrack using music that's already on your computer, and assign different music to different events (e.g.: a fast-paced track for when your aircraft are dogfighting UFOs, a calmer track for when you're waiting for alien activity in Geoscape mode, a fairly low-key - but tense - track for when you're defending one of your bases from alien retaliation attacks, et cetera).
I dunno, it may not be feasible - but it would be cool.