I just keep coming back to play with this!
I have a couple of ideas:
The Super Mario Sunshine acapella soundfont and the Minecraft soundtrack instruments would make great instrument packs to play around with.
Fullscreen option?
Maybe also the ability to increase/decrease the octaves
on the piano with +/- buttons?
I am a dev with some background in music -
would love to talk more about this with you.
Everything you've done up to this point is solid, thanks for putting it online!