I think that your modifications are headed in the right direction! You seem to have a clear goal/idea in mind, and I'm looking forward to seeing how you flesh out the game to achieve that.
The best advice that I can give you in terms of level design would be to make sure that each level has some sort of unique identifier. New types of challenges, enemies, obstacles, or powerups can massively help with this. Putting some time into playtesting and differentiating your levels (to figure out, from a player's perspective, how the level plays) can also give you some direction in how to design things.
I'm also looking forward to hearing the music that you create! That always adds a new layer of depth and significant change.