Separat channel (not sure how else to call it) for the stream music. So I would be able to mute stream music without muting the streamer and everything else (if the streamer does his setup right ofc). Would also help with muted videos due to copyright issues I could imagine.
This forum is for third party developers that integrate with the Twitch API. I would suggest https://twitch.uservoice.com for suggestions like this.