I agree with sensino on most things, and I have a few ideas of my own;
maybe make it so after each world, you have an option, either a new note, or a new column for notes, that way it slowly becomes more complicated at a manageable rate, allowing you to slowly proogress towards having a massive symphony.
you could possibly make it so that bosses drop heart upgrades, so you go from 3 to 4. this would work well
I really enjoy the concept of special attacks, like the riot shield, it adds a unique spin and keeps it interesting.
this is an amazing game and I cant wait for it to be fully released, it is such a good concept.