I think that these modifications sound great! A tip with making new levels is to make each unique and special - new mechanics, obstacles, and challenges can help immensely with this. Making your own custom sounds and music can add a lot to a game, there's a lot of tools online to aid with this, I'm excited to see what you make! What type of enemy variation are you thinking of making?
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I like enemies with infinite health that take advantage of the player gaining an additional jump after jumping on them. My goal is try to create some kind of poison/acid element that damages the player on contact, and make a focus on jumping on enemies to cross gaps. I hope to be able to figure an infinite enemy spawner as well