How bout a small planet-like area that loops, and you have to do something to get out? Or an island in the sky that you have to do something tricky to get to? Or a waterfall cave (a cliche, honestly)?
I think I'd like to see a glass tunnel, or a reef, or a ruin that is surrounded by water/partially submerged and is more platforming than traps or puzzles. Thos last few are ones that would be cool to see.