somehow be able to continue to edit playlists after sharing with other people, which would update the people you've shared with! Presumably it'd have to be stored somewhere in the cloud, and everyone would need to sync periodically

e.g. singer sends her set list to pianist and bass player by emailing a playlist. The next day she wants to add another song so adds it to the playlist... and the song is also added to the playlist for the pianist and bass player (without her having to send it again).