I've been thinking of and briefly working on a similar project.

One idea is to analyze timing as well, and "trigger" things after certain sequences (so play 1-3-5 as say eighth notes and then get an in-rhythm arpeggio one octave higher) or detect the beat and play on the upbeat.

I haven't done any Rust, but this might give me a good reason to give a try.

Have you considered making it a plugin? (makes replay easier in my opinion, but brings other pain like relaunching the DAW between builds...)

I had added a metronome and a note generator to this earlier which aren't working well exactly right now. But this would definitely increase playability. I agree that there should be a VST plugin version of this as well. It can also live as a standalone app and a plugin. Could you elaborate on the analyze timing? If you had to play using this, what would you like to hear ?

created https://github.com/contrapunk-audio/contrapunk/issues/8 https://github.com/contrapunk-audio/contrapunk/issues/9, let me know what you think