What you describe is exactly what I'm building - https://songsling.io

Didn't want to explicitly promote it here in the thread, but I'm building it to serve as a general music-content engine for artists (also for my own music, let's be honest) that will let you upload composition parts and the player will put them together on the fly. It already has interactivity built in where you can serve a different version of a track based on changing inputs you connect from your Internet presence etc. Plus, you can DJ with it, it supports smoothly changing playback speeds. It needs to happen.

One use-case that immediately jumped to mind for an API plugin is integration with popular games for game streamers to use. Something that detects that you're in the middle of a gunfight vs just chilling in the lobby, and selects/modifies the music appropriately.

Some video games do this automatically with their in-game music, but not all, and many streamers prefer using other music to avoid it getting stale.

Just wanted to say that this looks super dope. Love the aesthetic as well.