I am using Spotify Folders to effectively amalgamate "playlists of a kind" that I am following or playing regularly. In this case, I am listening a lot to the so-called "yacht rock" or guilty-pleasure West-Coast-style soft rock and similar music of the late 70s and early 80s and find that most of the Spotify playlists will have the same songs but there will be some other extra songs that one or more playlist contributors have added. But most of these playlists have songs that are common to each other, even though they may be derived from different albums.
A feature I would like to see for "folder-wide shuffle-play" would be to identify songs that are common across the playlists in that folder and see these as one song. There could be the ability to differentiate between variants of that same song like an album version, 12" dance mix, radio edit / 7" version or live recording and allow these to be seen as other songs. The goal would be to stop one song being played too many times during "folder-wide shuffle-play" because it appeared in many playlists in that folder, but if there was the "live cut" for example, Spotify could play that as if it is another song.