Having a selection system along with some sort of a filtering system in playlists, I think would be a huge quality of life change because it would make things just a lot more easier. It's not that complicated of an idea but basically what I mean is that how about a feature where we could select more than one song at a time when adding songs from playlists/liked songs to other playlists, removing songs from certain playlists, and adding songs to queue, because as of right now it really feels very difficult and time-consuming (sometimes even frustrating) to add many songs at once because of how each should be handled individually. And as someone who has and does do this, it feels like a missed oppurtunity.
Also, I think along with this, adding a filter system where we can filter songs that are: duplicated, appear in other playlists and have explicit lyrics and such, would just add on to the selection feature because combined they both feel like they would not just make the app better but the user experience a lot more efficient and useful!
I know that i kinda included two ideas but i feel like they both go very much hand-in-hand and that they kinda complete each other in which it eliminates the hassle of handling a lot of songs at once and makes keeping track of what i listen too and my playlists a lot better!