I think that your proposed modifications sound good, I have a couple of comments:
- Will the backgrounds that you change be custom-made? This is a great opportunity to hone your artistic skills and put some time into significant asset updates.
- As a tip, when making new levels, try to make sure that they're significant and unique/different from others in some shape or form. Adding new enemy types, challenges, or obstacles can help with this!
- Are you proposing increasing the quantity of already existing enemies, or making new enemies that have custom abilities/stats? I'd highly encourage the latter!