I'm currently working on a DAW, and this is definitely the direction we'd like to head. In our previous version, we added some basic chord progressions. The next most likely chords are suggested on statistics from popular music. But I'm stuck on where to take it next. I have some ideas for a melody generator, but I don't really know anything about machine learning, etc. When you are ready to implement you API, I'd be very interested in talking to you. Send me an email at roland@tracktion.com