I've been looking for something like this, here are some suggestions:

- system-wide keyboard shortcuts (I believe there is a Carbon API for this that you could use). It'd be great if you could hook the default Pause/Play buttons on the Macbook keyboard, not sure if this is possible

- possibly some kind of current track menu item (I wrote one for iTunes here: http://silentmac.com/?p=470 ), requires Cocoa. Just to display the current song (some of my music has long, quiet intros, and I'm not sure what song is playing in the beginning), this might go against your idea of minimalist though

- shuffle functionality

But it looks great so far!