Back in the days of iTunes and iPod, i loved to comfortably manage the music on my iPod via iTunes on the desktop.
Since data plans in Germany are capped, i need to set music i want to listen to on my phone as available offline, and download it via WiFi when i'm at home. Currently i have to scroll through my HUGE music list on the mobile phone, and click on every artist, or album, or track, or whatever and flip the switch. This is not very comfortable.
I'd like to have a feature where you can select a song on the desktop, which is already saved or added to a playlist, or is in some other kind of markable as "offline available". When you select this song, you will have a button like "Sync to your phone", and when you click it, the song is queued. The next time the Spotify App on my mobile phone is started, it compares its list of offline available tracks with this queue, and downloads additional songs which are in the queue but not saved offline, or deletes songs which are available offline but not in the queue.
Together with some special display element which clearly shows a song as downloaded on the mobile app, this would also reduce confusion when you have made a song available offline via its artist view and via its album view, and you want to remove it from your phone to save disk capacity, but it is not really deleted since you only unchecked the artist of this song in the artist view, but not the album of the song in the album view.
TL;DR: I want a comfortable option to manage my offline available tracks on my mobile phone via the desktop app.