Just a quick “Wooohooo!!” for Spotify playlists, too!!! I make playlists for Korean, and this would help so much!
Rooting for your hard work! 
I’ll edit this reply if I think of any additional ideas!
P.S. Quickish Question: Just thinking about the playlist-to-LingQ-course feature, would there potentially be a way in the future to add lessons to courses (through this feature) or specify specific songs/videos (e.g., popup with a multi-select check box or something to select videos from the playlist) within a playlist if users update their Spotify playlist/Youtube playlist—if it’s an ongoing list?
Edit 2: Found this post on individual “import” of Transcripts for Spotify (thanks for you great reply to that one @roosterburton!).
So, users can import those new additions through the LingQ importer if they don’t have too many, but if they have a bit more, it might be an idea to consider later down the road!