I like the modifications that you're proposing, nice job! 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.
How're you planning on making the new enemies function differently from the base ones? Will their stats be tweaked or have entirely new abilities? I'm excited to see what you come up with for your new player mechanic!