Over a decade ago, long before Spotify started, I was an avid user of last.fm's radio, and I loved it's feature (bug?) that it regularly crawled away from the original band or genre set to play: it was a fascinating way to discover the broader similarity of artists to the one started it.
It seems reasonable simple to implement: the upcoming track should be based on the "radio" of the current song playing, and not the one the radio was originally started with.
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