Your proposed modifications sound like they're headed in the right direction!
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!
With new art assets, this is a great opportunity to learn a new skill or hone your existing skills - I'd very much encourage you to try and make your own assets. Plus, it would be a lot more significant/unique of a change.
When you mention adding more enemies: Are you referring to increasing the quantity of existing prefab enemies, or making new/custom enemies with different abilities? I highly encourage you to try the latter!