I think it would be interesting if plugins could be written that could modify how the playlist and possibly the toolbar on the bottom is displayed to the user.

I mean, in highly graphical, unique, and fun ways. To present the information about the playlist and column items in unique ways, such as displaying cover art in groupings of albums, maybe modifying the size of a displayed cover based on what percentage of songs you have from it. It could also be interesting to incorporate song queues, equalizer, and even visualizations.