When creating a playlist it can be time-consuming to individually select each song from your library or another playlist, so there should be an option to select multiple songs at one just by clicking once on each song. Then there should be a remove from playlist and an add to playlist button when the user is done selecting. This would make it much quicker to create and edit playlists.
Updated on 2019-08-16
You can already do this by holding Control (Windows) or Command (MacOS) and then left clicking songs. Songs you click will get highlighted. Hope it helps!