I use Plex for everything and I would like playlists to be improved upon, for music specifically. For instance, syncing playlists on the iOS app doesn’t show up on the playlist page of the app. I believe that’s because I already have all of my music sync’d and if so that should be fixed.
Enabling the ability to create playlists on the iOS app is also something that should be looked into.
And lastly smart playlists, much like the how Apple offers on the itunes app would be awesome.