THIS GAME IS SO AMAZING!!! I gave you 5/5 in every field.I'm a little biased because I play the Piano, But I think you should DEFENENTLY make another game, where you keep the movement and the music mechanic and add more story and character to it. If you want, you can make a steam release out of this main idea. I AM BEGGING YOU! Please make this into a bigger game. I would 100% buy it no matter what. I'd buy it on launch day. (Tough you still need good marketing for this, I think that the main idea combined with a story could be a really good game.) I imagine this game like Once upon a coma/Neversong. There are some games like this, but there isn't much. So I think that you could set yourself apart relatively easily. One unique selling point (USP) to the game could be that you have the ability to play anytime, you want (not just when in a specific room or something like that similar to the other game I mentioned)
Important: please add the ability to keep the buttonse pressed. It would add so much more depth if I could use chords and such.
What I would change: Please add arrow controls because, that way I can use my right hand controlling and my left hand playing the piano. Wasd keys are only good if you're using the numpad for playing, but 1: I don't have a numpad and 2: playing on the numpad isn't piano-like anymore, because there are keys above each other.
BTW if you could check out my game that would be cool, even though your's is superior, it is my first complete game. :)