I think that you've come up with some solid modifications - nice work! I'm looking forward to your custom art assets and seeing how your new enemy works with custom scripting. Good luck!
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.