I'll give it a shot.
- I have always found the shape of a bottle is really cool. Plus a level set on a Ship in a bottle would be really neat to see.
- Some surface that is like a cylinder, so you can walk up the side and then be standing on the ceiling, and then walk around the other side. That'd be neat.
- Somewhere where your character is very small and a rainstorm is happening, maybe through a "forest" that's actually just grass, and the raindrops are huge.
- A keyboard level would be super cool! Like your character is going through a maze but its actually just the spaces between keys on a keyboard. I got that idea as I typed this up.
That's all i got, hope it helps!