I'll admit it --my musical tastes are specialized. I love a broad range of classical music -- and not much else. So I see programs like Clementine through that lens. There are two things on my wishlist that would be invaluable for classical music, but almost certainly far more widely useful.
1. Recursive playlists. This is an obvious enhancement and probably not at all difficult for someone familiar with the procedures for modifying Clementine. The idea is simple: an item in a playlist can either be a song or another playlist. My favorite example of a natural hierarchy in a library:
Beethoven -> String Quartets -> Early Quartets -> Quartet #2 -> 3rd movement
But for a different domain, we might have:
Performer -> Tour -> Concert -> Song
You get the idea.
The pseudocode:
procedure play(item):
if song(item) then perform(item);
else for each x in item do play(x);
end;
2. Configurable genres. "Classical" is far too broad a genre; at least there should be a subgenre such as Oratorios. Every other musical genre will also have subgenres.
Ten years ago I would have taken on this project myself. Alas, I'm too old now to be able to do it. I'm hoping that someone in the Clementine community will be inspired to do what I am no longer able to do.