This is a theory I've just come up with. Probably won't ever happen, but I thought I'd share it anyway.

It's about the Cage and Michael and Lucifer.

We know the Cage was built to withstand the strength of one archangel, right? That's why the seals needed to be broken to unlock it before the Apocalypse could begin. And for the first year and a half, Michael and Lucifer were probably too busy torturing Sam and being annoyed at him for wrecking their fight. Then they probably did fight for a while. They're probably doing that right now.

What if they eventually decide to have a ceasefire and ally temporarily in order to break free of the Cage? The strength of 2 archangels together could be enough to do it.