A browser extension / additional service that allows you to tag/lookup the music on google music / spotify by any set of strings. And I mean your own saved collection, not for sharing/exploring/social purpose. This can be faked to some extent by multiple playlists, but it's a bit hard to use in real life.

This would be great help when trying to dj dancing party: now show me a list of "blues", "chicago", "slow", "with breaks" to choose the next song. Actually in this context, separate service / app would be great - it would work on a mobile device, rather than just desktop.

(a long standing idea on spotify list already: https://community.spotify.com/t5/Spotify-Ideas/Tag-Music/idi...)