That's so interesting - you're totally right that setting constraints often leads to really creative ideas. It reminds me of the "crab canons" by Mozart and Bach: https://en.wikipedia.org/wiki/Crab_canon .
I also think there's room for other creative encodings for music - possibly expanding these notewise/chordwise ideas, or possibly going in a totally new direction. It's fascinating to me how much the generations are affected by the encoding.
Another fun direction is to generalize the kinds of constraints we put on our own instruments! I had a chance to play with that in a graduate class by implementing an API for midi generation where you set chord fingerings and strum patterns independently for a guitar of [N] strings.
Of course, I had to "play" the guitar myself by writing song sequences in those terms... it would be terrific to see what an AI could do with a notation scheme representing, say, a 20 string guitar or a 30 foot long flute.
Baudelaire said something like that (about the sonnet): "Parce que la forme est contraignante, l'idée jaillit plus intense" (poor translation: "because the form is constraining, the idea comes out more intense")
I also think there's room for other creative encodings for music - possibly expanding these notewise/chordwise ideas, or possibly going in a totally new direction. It's fascinating to me how much the generations are affected by the encoding.