I would find it fun to play through the campaign with a friend controlling the enemies, or the other way around. It would also be a good way for a someone to drop in in middle a game and just have fun for a couple of hours, while the one controlling the characters can still focus on the story that they might already have invested 50 hours in. It's pretty hard to find the time to play through an 80 hour game with a friend.
I really don't think that will happen. A GM messing with the main quest scripts and triggers might break them.
Honestly the idea of dropping into a random game in this kind of game does not make sense, much less dropping into someone's random game and becoming a GM.