I've just spent far too long writing the beginnings of a ridiculous music game for web browsers. You use either a MIDI keyboard or the computer keyboard (a s d f g h j k). The game plays a bar of random notes and you have to play it back, correct notes and correct timing. As you level up, the patterns get more rhythmically complicated. If you make 5 mistakes, game over; you have to start again. Still probably a lot more to do to make this more interesting, but I'm curious as to whether anyone finds this remotely worthwhile.
One challenge right now is how to calibrate if you don't have absolute pitch. I feel like I might need to add some sort of screen where it plays you a few notes until you find the right ones or something. For now, note that it's in A minor, A3 to A4.
https://files.jantrid.net/simus/