User blog comment:KillRoy231/Ouroboros theory/@comment-1031724-20190328035639

Or, new headcanon if Season 15 and the rest of 14 disappoint:

Jack heals his soul with his archangel powers. Castiel tells him of Heaven's imminent collapse and Cosmic Entity about to kill him. Jack goes to the Empty and kills Cosmic Entity to save Castiel, thus preventing Lucifer's revival, then he goes to Lucifer's cage (powerful enough to not be imprisoned inside), rescues Adam and heals his soul (preferably erasing his memory of hell), traps Michael in Dean's box, places it in Heaven to save it.