At one time I wanted to make a theme without using any rectangles. I'd use transparency to cut the edges off of everything, and something like grdMagick to distort things like the VWM so that the miniwindows and such weren't square. (except, there is still no such module... :(

How about something that looks like a city skyline across the bottom of your screen? That could be kinda cool.

Or, another one I wanted to do was a theme based on pies. Apple pie, peach pie, etc. Mmmm, pie. Of course then you'd need pie menus as well. (all my best ideas need new modules unfortunately...)