First off, thank you for this music player - it's brilliant and is taking the place in my heart previously held by Amarok of old.
The one thing missing for me is to be able to label tracks arbitrarily. The sort of system where there is a set of possible labels S and each item in the collection belongs to labels in a subset of S.
For example, some tracks span multiple genres simultaneously; instead of generating a cross product of possible genres and placing those in the genre tag, I would prefer to label it "rock" AND "blues" AND "electronic".
Playlists can function as a sort of label since a track can belong to an arbitrary number of playlists. But playlists as a substitute are less feature-rich than a labeling system could be. For example, a smart playlist could include the union or intersection of two labels. (i.e. use boolean operators on label membership.) Additionally, in the current interface, creation of playlists is a high-friction process: there is no way, as in the Spotify player, to right click and instantly create a new empty playlist.
Labels would need to be stored in track metadata like any other tag. A simple comma-separated list would probably do.
I guess that's the gist of it. Thanks again for this great Linux music player - it's so nice to have a powerful interface to work with!