I like where your proposed modifications are headed! Keep in mind that they should be significant and take some time to complete. With visual assets, like you're thinking about doing with the main menu, I'd highly recommend making your own artwork, it's a great chance to hone your skills!
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!