Add the ability to edit, import/export, and backup (locally) songs (just the track as text record, not the actual song data), playlists, and metadata, including liked songs and associated metadata. This would at minimum include Date Added metadata.
The reason is that sometimes accidents happen with the liked songs list and you can easily lose many years of "date added" time stamps, and for people who use it like a musical diary it can be devastating to lose that information. This can similarly happen with even just one song, where sometimes you accidentally remove a song from your liked songs or other playlist where the date added had special meaning/significance in the context of your life. Having the ability to at least re-add that song and then edit the "Date Added" metadata would allow people to recover from that error.
Operationally:
You could limit the 'edit' ability to desktop-only, I don't think that would present any issues for mobile users. You could right click song, then 'edit metadata', and change the 'date added' to whatever is the correct date for a given song.
For backups, you could periodically create local backups that include all songs, playlists and all metadata, including the liked songs list. A restore operation would then restore all previously backed up songs, playlists, and date added metadata (including liked songs and date added metadata). Bury this in the settings menu or something, I can't imagine this would take a huge amount of space since you're not actually backing up the songs themselves.
Imports/Exports would work functionally the same as backups but would instead be on demand rather than automated like the backups system. If you're worried about people using this as a way to change music services, lock it somehow to Spotify-only or something, but this would at least allow people to know they have a solid backup permanently available locally that *IMPORTANTLY* includes metadata like Date Added, in case something happens to the liked songs list or some other playlist of significance. An important differentiator between this and a 'backups' system would be that these are separate user-driven backups that are not overwritten automatically like I imagine a rolling backup system would work. This would simply be a 'backup' but from a specific point in time that someone could always fall back to without fear of it being overwritten.
I think this would be a welcome addition and only enhance Spotify as a service, and you'll have hopefully prevented a lot of future heartbreaks from silly errors.
Thank you