I think you're headed in the right direction for your modifications, but I have some questions:
- How are you planning on making each of the new levels that you create significant and different? 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.
- What types of new enemies will you add? Will they have new abilities, sprites, powers, etc.?
- What types of obstacles and challenges will you add?