The now playing list. Once we add something, the only way to reorganize the list is by deleting, and re-adding. Perhaps a button on the left hand end of the son in the list that you can grab to move the item up and down in the list. This way we can reorganize the order of files within the now playing list.
This would go a long way in giving us a better way to handle playlists while on the player.
Another thing would be the ability to long press a song in the now playing window, and have one of the items in the menu be Add to playlist... which could then open a dialog where you can pick either a current playlist (which it would append the item to the end of, and then we could reorganize it later with the first item i posted) or pick 'new playlist' which would then open a dialog where you could type the name of a new playlist.
Another item that would be nice, is perhaps a way to get to playlists, and be able to modify them (while music is playing, not from the playlist) a screen where you pick the playlist, and it opens it up with buttons along the bottom(delete(or instead of a delete button, a swipe to left or right to remove an item), shuffle, add), and the checkmark boxes on the songs, like when you select songs, as well as the item i mentioned before for rearranging them. The add button could let you go into your library and start adding songs to the playlist, of course this would be redundant if you could simply long press a file in the normal ui and say add to playlist.