Some ideas that would be fun to have in future revs of EMPEG visualzations:
1) Sonograms. The X axis it time, the Y axis is frequency, and brightness is amplitude of the music at that time/frequency. You scroll at some speed, maybe one screen width per second or two. A speech recognition friend of mine spent an awful lot of time looking at sonograms and I found them quite pretty, although they look better when you have more than four levels of brightness (or false color). This should just be a hack on the existing spectrogram visualizations.
2) Cheesy video games. You could hack up Galaga or Asteroids pretty easily. The bad guys / asteroids could move in time to the music. The "player" could either be robotic, or could be driven from the remote controller.
3) Video feedback with song information. As the track information is rendered, you could do standard video feedback stuff on top of that, giving the rendered text funky shadows and stuff. You could use the music to drive the angle and direction of the video feedback.
4) Clocks. Analog clocks. Digital clocks. Tux-the-penguin clocks. Dahli clocks that morph with the music.