Free idea because I’m never going to get around to building it: An “AI 8 track” app; click record and hum a melody, then add a prompt and click generate. The model converts your input to an instrument matching your prompt, keeping the same notes/rhythm you hummed in the original. Record up to 8 tracks and do some simple mixing.

Would be a truly amazing thing for sketching songs! All you need is decent humming/singing/whistling pitch. Hum and generate a bass line, guitar lead, strings, etc. And then sing over it - would make solo musicians able to sketch out a song far easier than transcribing melody to a piano roll.