It would be nice to be able to create smaller playlists within a playlist.
So you would still have the main playlist with all the songs you've added to it but you can then sort the songs already in the main playlist into other smaller playlists within the main one and you can choose which smaller playlists are playing within the main playlist.
So lets say you have a playlist called XYZ and you have 100 songs in that playlist but you want to organise those 100 songs into 5 smaller playlists within playlist XYZ. Then lets suppose you just want to listen to the songs in playlists 2 and 3 from those 5 smaller playlits, then you would choose playlists 2 and 3 from playlist XYZ.
So you would be listening to the songs in the main playlist but you have the ability to make a more narrow selection of the songs you want to listen to but you would still have all the songs in the main playlist.
You can still make as many main playlists as you want obviously but you would also have the chance to organise the main playlist. This means you wouldn't have to make separate main playlists having to add the same song several times. I believe it would be very helpful.
As mentioned, playlist folders are already available in Spotify. In the Desktop app, right click in the playlists area, and you'll find the option to create a playlist folder. Once created, you can drag and drop playlists to the folder for better management.