astro

I think that your modifications are headed in the right direction! I really like your idea of adding music - are you planning on making it yourself? I'd highly recommend doing so, as it makes the game a lot more unique.

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.

In terms of adding enemies, I'd recommend trying to make some new enemies, rather than adding more of ones that already exist. Perhaps try seeing if you could create some new abilities or behaviors for an enemy, and then use those in your levels - just to differentiate things?

I'm looking forward to seeing what you come up with, good luck!