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I like where your proposed modifications are headed!

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 adding visual and audio assets, this is a great opportunity to learn a new skill or practice an existing one. I'd highly recommend that the background decorations / music you add are custom-made, it'll look a lot more unique and be a more significant change.