Spotify Extension , Amazon Music
What I was thinking was something similar to the "Watch Parties" but for music. Idealy what would happen is your would require a user to sign into their Spotify/Amazon account and click a link that links outside of Twitch to listen a long with the streamer. I think this would be cool and an easier way around the DMCA. You would not be playing the music over the stream however behind the stream itself. Just an idea I thought was cool.
There's an Amazon Music extension that does this.