i also thought of making a theme every time you complete 5 levels or dungeons, the first five are normal, then you seem to 'shrink' in the next five, making it harder due to giant monsters and giant objects. as if you were a mouse sneaking into a giant zombie infested castle ruins. then the next levels are in the nether, allowing for booby traps such as super fast lava ceilings. i also would make solveable puzzles kinda like zelda, but with added things, like parkour. even minigames where you have to beat a mob arena to pass.
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