How about a barrier-based parkour? Because terrain in Platformer mode is genuinely awful lol, you can also make one type of jump technique for each summit.
How about you make a parkour shooter game, where you try to collect key cards from enemies that drop them. Maybe you can make it so that every 5 levels there is a boss platform, and every 10 levels the scene changes. You could also make a shop where characters buy different boosters, such as speed or health, as well as different we@pons. You can make each level harder and unique in its own way! I hope this helps, and if you need more, just tell me!
maybe a game mode where you have two overlays, one to activate platforms and one without. this isn’t the theme (this’s been used before), but it serves as an interesting twist to the platforming.
maybe try a slope jump dropper where when you enter the level, there’s a timer. once it reaches a certain time, the exit to the game appears, but the issue is that the zones are closing in. the starting area will be unavailable and the floor isn’t able to be walked on, so they have to continually hang onto the slope jump until the door appears. i explain things badly, sorry bout that
drop jumps, but the middle part slowly gets more tilted and the drop jumps harder, until the drop jump is impossible and the tilted middle is a slope jump, which is easily able to be jumped over. basically a succession of drop jumps ending with a secret slope jump