it's a great idea
you should try adding the whole spectrum of piano notes, or at least more than one octave. It would also be cool if you allowed for keyboard use instead of just having to use the mouse. Being able to control how long the note is held for would be nice as well. Putting these kind of things in version 2 would increase the overall score since it would feel more like playing a piano than version 1 ( assuming you are making a version 2 that is )