This game is actually so fun, lol. The songs are fun (both the sound and the play patterns), the difficulties are impressively balanced too. It seems like you had a lot of good playtesting sessions, or your intuition is spot on.
I think adding a little bit more juice when the player successfuly hits a note (a sound, or a visual effect, or something) would go a long way - maybe the visuals could be improved generally. But overall, the game is already really fun. I definitely think I will be coming back to it sometime and trying to master the highest difficulty, lol