I know this will never happen, but I'd like to see SPC and PSF support (ADX support would be nice as well. Since I'm dreaming, I'll mention that). SPCs are emulated audio from the SNES. No matter how long the song is, it's always 65k in size. I've seen SPCs that are more than 12 minutes long. Also, they loop infinitely. So, to reiterate, this isn't going to happen (SPC support), but it'd be pretty neat if it did. Useful as well.
ADX support would be nice as well, but that's even more unlikely. Unlike SPCs (which users can't create. They have to be pulled from a SNES game), ADX files can be created by anyone, and they have looping support. The problem with ADXs, is that they're too big. Realistically, a 500k ADX is not going to amount to much. Though, a mono ADX will certainly afford more than a stereo one.
For code and info on SPCs (and a huge slew of emulated audio) and ADXs, you can check out
Game Music Emu (for SNES, etc.) and
in_cube (for ADX. The source is available on that page).
As I said, this isn't going to happen. Still, one can dream. I'd like to see it, but if it ever did come about, my jaw would drop. Only on April 1st would I even imagine such a thing might happen... Still though, I provided some useful information, in case it interests anyone even in the slightest.
(more) ON-TOPIC:
As for MOD, IT, XM, S3M, etc. support, I'm all for that. Support for that is *far* more likely. I think it'd be a good idea. It allows for smaller files (smaller than 500k), and superior audio. And, if done correctly, I think it can provide looping support. So, please, feel free to ignore my dreams (above), and instead focus on MOD (and etc.) support. I see far more use, and functionality, out of that one.