Stuff I wish I had:
-Playlists that worked in trees. As in, multi-level organization. (one playlist inside another)
-More fuzzy searching for files. For those of us who have so much music, most of it is on an external volume - if we move files, it should be able to hunt them down. Somehow. Hah.
-Active checking for files. If they don't exist, they don't exist. If they do, don't _forever_ mark them gone. Or at least implement a "rescan library for missing files" option or something.
-Adding ratings to a file should save it to the file in the comments or something, so if I decide to reimport my library, I keep my ratings.
-It should be an option to save data to the mp3 file itself, or to save it to the library. The former option is bad because it messes with the file, making it less compatible with file sharing hashing. The latter is annoying because you can't do a massive wipe and re-import. And you can't just save the data off to another smaller file and keep it by the mp3 file - that'd just be plain annoying for people who have massive amounts of music. No one wants, 30,000,000 1K files in addition to their mp3 files.
-Plugin support. For OGG, FLAC and other formats.
-Better searching/filtering options. What iTunes has is nice, but it's not powerful enough. We can't search comments, we can't filter by other stuff.
-Interface. While I consider this to be the most annoying things about some of the other MP3 players (audion's skins... oi~ some are just plain ugly, sorry.) the option to skin your interface is probably nice. And it'd be nice if you could chose whether you want it all broken up into separate windows like WinAmp, or if you want the single-window interface like iTunes.
-Web-based interface and music broadcasting? XD
It'd probably a really ambitious project, actually. Considering how I still haven't written any GUI-reliant applications for MacOS X. Or for any platform, for that matter.
Suggestions and ideas welcome.