Something that's been mentioned before, and would probably form a significant project, is to provide a (text) scripting interface.

This would, for example, allow users to iterate over all the playlists in the /albums playlist, and sort them in tracknr order (to use an example that's been requested recently).

Scripting would allow users to add this kind of stuff more easily than hacking the Java code, leaving you free to code more cool stuff.