Maybe we can generate a list of lessons/courses where improv is taught on real songs as opposed to general improv tips.

I think it will help because often we learn general improv tips but don't know how to apply them to songs.

I personally would prefer to learn improv on rock,pop and classical songs and maybe,even on worship songs(as those tips can be carried to rock/pop) songs.However, we can make a list on any song.

I would like to start the list by pointing to the following 4 improv lessons (Btw,quaverbox's paid $5-$7 lessons are awesome,try them) on real songs :
https://www.youtube.com/watch?v=VhynjSOSZZw
https://www.youtube.com/watch?v=c2LVBXOkiXs
https://www.youtube.com/watch?v=NIHKSt0Q92c
https://www.youtube.com/watch?v=WCYfud03cgE

All the songs are by the band Hillsong.