I enjoyed playing your game and thought the idea was ingenious - I sincerely hope you consider creating a post-jam version.
I think that having an organic sanity/scoring system that rises and falls with the performance of keys is intuitive and helps ensconce the nuts and bolts of game mechanics in the performance of playing the piano. But I get that long-term play might need something more.
Agree that melody could be an opportunity to build the musical aspect of the game experience - perhaps harmony and discord could be used to reinforce the player's sense of success and failure states?
Sometimes the simplest concepts resonate - pun intended - so I'm glad you got a lot of positive attention through the Ludum Dare. I liked this. It was very thoughtful take on the rhythm action genre. Great work!