Sounds like a good start. You should also push all the playing music into a "Playing Now" play list so that while music is playing, it is possible to add songs and thereby create dynamic playlists on the fly.
For adding music to a playlist (either playing now or a saved playlist) it should be able to add music to the target playlist array using"
1. Add all music by an artist
2. Add all music by an ablum
3. Add all music by folder
4. Add all music by genre
5. Add a single song
6. Add all favorites
From a UI perspective, holding a designated key down (like play) for longer than 2 sec would pop up the "send to playlist" options that would allow you first to send to playing now, and secondly to send to a named playlist or create new playlist.
It should also be able to browse to "playlists" and send one playlist's songs to another playlist, allowing you to save "playing now" or merge one playlist into another.