App someone should write

While playing my music files on shuffle, it occurred to me that it ought to be possible to have a database somewhere that lists the themes/topics/moods of various song. and hen an app that'd scan your music library and play songs in a "random" order where each had something common with previous one.

Sort of doing a thematic random walk thru your library.

Maybe options to look back more than one song in choosing the next so the playlist flows better.