I need more of this, immediately. My suggestions:
1. Fix the hitsound volume slider, it doesn't seem to work. I want to be able to hear the music without the clicking.
2. More doodles on the maps! Holy crap I LOVE those, they're so cute!
3. Speaking of the doodles... would it be possible to add optional "instant death" notes that have the doodles eat you if you fail them? Was inspired by the first few doodles on Track 4.
4. Some menu music would be nice, as a good way to warn players they may have their volume too high or low before they get into a game.

I would honestly put down some money for this if it had more songs. Wonderful game!