One idea I have for how to make this game a little bit more interesting is to have different instruments you can get.
Instead of starting with all 4 of your note placement lines, you instead start with 2 basic ones like a drum kick and a snare. As you explore, you find more instruments with various effects and you can mix and match them together to get a perfect combination. There could also be BPM increase items, which simply boost how fast your rhythm will play, and even offbeats where you could put one extra beat in between 2 to fire even more beat bullets.
There is so much you can do with this concept, I can't wait to see the final product!