I always really enjoyed the artwork on GTB late 70s/early 80s stuff. I'm thinking Asteroid Annie and the Aliens. Always thought that was a beautiful backglass and PF. You can get really creative with space/cosmic/alien/futuristic sci-fi themes. Can pretty much take it in any direction you'd want to go and it would be all "believable" in that world.
I love fantasy themed things too. Mythical beasts like dragons, barbarians, wizards, etc. It might be going into "nerdy" territory, but one could argue that the majority of pinball enthusiasts sit in that category anyways