As a electronic music producer, I would love to see a game mode with the BPM vamped up - most likely an expert or hard mode! I love listening to the cool beats I end up making in this game, but the rhythm seems a bit sluggish, so I think a hard mode with stronger enemies and more difficult bosses (and of course, a faster beat, maybe 128BPM?) would be a great addition to the game!
Overall, I think this game is amazing and very unique. Your game has a lot of potential and could be a future banger!