As you build up playlists, suddenly they are full of music that you like, but are very different.
So, you can be all pumped up with a rock tune and then suddenly shuffles to a mellow mood kind of tune. It would be great to have a shuffle mode that follows the mood of the current song. Basically adapting the "similar music" functionality to make it work within a playlist. You can even throw in some new stuff automatically. Similar to Last.fm's Mix stations, but not quite.
This idea is similar to a couple already existing in the community: https://community.spotify.com/t5/Spotify-Ideas/Search-by-BPM-beats-per-minute/idi-p/335 and https://community.spotify.com/t5/Spotify-Ideas/Add-BPM-to-content-metadata/idi-p/418002 .
Please add you kudos and comments there.