I would really like to see a shuffle mode for Spotify playlists.
I don’t know what else to say, so this is it 
I’m not too familiar with the Pi Musicbox webclient, which I assume you use, since you post in the Pi Musicbox category, but Mopidy in general can both shuffle the play queue, so that you can see the new play order, and play in random mode. Random mode keeps the play queue order, but selects the next track by random.
How to shuffle or enable random mode varies from client to client.
With Pi Musicbox webclient click on Now Playing at the top left (or click the album art in the footer) and then you’ll see three icons at the bottom right which control random, repeat and shuffle (in that order, left to right).
It works like that indeed, but it should be better… :-/