On The Flip Side

In one world, the apocalypse starts. John Winchester loses his sons and his mind with them. He picks up a baseball bat, he wraps it in barbed wire, and he releases his grief with every swing he takes.

On another plane of existence, the dead don't rise, and John is the one who dies. His sons mourn him. They keep him in their memory and try to do right by him, because even if he wasn't the best father, they both know he tried. And they figure that's a hell of a lot better than nothing.

Or: Two worlds collide when Sam, Dean, and Castiel drive through a rip in the universe into an alternate, post apocalyptic dimension, where there dad is a tyrant and the dead are walking.