I think a nice feature would be to make all tabs and panels dockable, so anyone can decide on its own where he wants which panels.

An example of the possibilities:

• You can keep your collection always visible in a dock at the top of the application.
  
  • An extension of the collection-dock/tab would be nice, so you can not only see it in a tree, but also in other structures (like iTunes) as per genre/artist/album in three separate lists. 
  
  
• You can also have the cover manager  (which also needs some extensions for this) at the top, so you have an overview of covers which are (going to be) played and you can choose your music from there (sorry but also a bit like iTunes).
  
  • maybe this functionality can be integrated in the collection-dock/tab and have it as a different view
  
  
• You could have your portable device always in sight, for example in the lower left corner, below your collection which is located on top of it, so you can drag and drop albums directly to your portal device.
• You can drag the lyrics panel/wikipedia artist panel/song info panel or anything you want to the right side (or any place you want) of the application to have it always on your screen, 

A new panel like the current music-context panel, where you can see info about the current song which is played (also with bitrate or anything the user wants), and where you can set the rating!, but without all related info, would be nice.

Also more customization for the prev/play/pause/stop/next - equalizer - volume panel would be nice, so you can change the size of the equalizer (smaller) and place the moodbar/progressbar next to it and enlarge this.