Who Framed Roger Rabbit?
Lord Of The Rings
I like the possibilities for both of these suggestions, nice work Jdurg.
A LOTRO Would be interesting indeed. Lots of spinner's that could be modeled as rings, you could have emulator skins where each emulation is one of the rings, with MAME being the ring of power.

Potential to make some kind of awsome side art/flashy art that could be mount doom, or Sauron's Eye!
Wow.....suddenly I have the urge to start sketching.....
****Attempts Jedi Mind trick****
You will forget the LOTRO idea, your not interested

I think another idea that I threw around, but never did anything with was a Mario themed cab, with all kinds of PVC Pipe routed all over/through it, painted green.
Transformers - I think this idea has a great potential for FE Skinning...where when you switch emulators, the transition animation is the autobot or decpticon symbol, like the old G1 cartoons, soundwave saying is phrases when selected, or Optimus...Autobots transform and ROLLOUT! Lighted Sideart with a plexi etched Transformers symbol that pulses with sounds of the games.....HAWT!!! Player 1 is translucent Red, Player 2 could be translucent purple.