Actually yeah, some boss ghosts to shake things up at certain level thresholds would be cool.
-The robot ghost from Arrangement is an obvious one.
-Toc-Man you could feasibly do something with by having him walk around slowly but occasionally shoot dots, rev roll and butt bounce.
-Biggaboo from Pac-Pix could be an interesting case where he hops around and gets bigger each time you chomp him with a power pellet.