I like the way your modifications are headed! I think that a new hazard, such as dripping acid, could be a great way to change up gameplay and add some new obstacles.
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 also like your idea of a new pickup! Best of luck with finishing up the project, I'm looking forward to seeing what you create!