I have been thinking and I honestly want to suggest a moon-based level, with less gravity. You said that this game in the future will have multiple worlds to travel to, so why not the moon!
Actually, let me just say what kind of levels there should be:
-Water levels, where you get bubbles for air...
-The moon, where gravity is less, but you move slower
-Lava world, where lava is everywhere...
-Plantation world, where the red kobold tribe lives (Maybe use this suggestion and the following?)
-The Mountains (Underground) where the blue kobold tribe lives!
-The Lab, which is home-base and allows you to see your Gallery
These are the current ideas for your game. I often GM for games like D&D and Pathfinder, so I can usually give ideas, but not make them into a thing if they involve coding or things... sure I can do world design but I've just started that.