I'm not sure when this feature was added, but I just discovered it - and it's AWESOME!
As far as I can find, this feature is only available via the Desktop client, but worth dragging a computer out to use. When on a playlist, scroll down to the very bottom to find a new section called "Recommended Songs" with up to six songs. The songs recommended are, as the UI points out, based on the songs in your playlist.
You have the ability to drag songs from the recommended section into your playlist and the recommended section generates a new recommendation. There's also a Refresh button if you want six new songs.