If it's ok, I'd like to suggest a way of implementing playlists inspired by the way I use them in the PC. There I just drag and drop directories/files into the playlist window, so my suggestion is,

Create a second pane on the right for the playlist which can be resized by dragging a divisor, sort of like some filebrowsers work. You would select the directories on the left and add them to the playlist. You would also be able to select items on the playlist to remove them. Add a simple shuffle and repeat button and I think that's all you would need to make playlist management way easier than on the stock media player.