Disclaimer: I own nothing but my ideas
Prologue - Rips in Space and Time
In the theory of the multiverse, it explains that every possible outcome, for every single action exists simultaneously, across different dimensions. Everything happens all at the same time. If a leaf falls in one universe, it won't in another. You make a left turn in one universe, a right in another, and in a third you go straight. Every possibility occurs. Some universe has a real TARDIS, a real set of Ravagers, a real Sherlock. In others, someone's imagination that made it up.
Everything always happens.
In this realm where this story takes place though, we deal not only with possibilities, but also specific locations. Earth. Heaven. Hell. Purgatory.
So, while Dean and Sam were looking in wonder at that golden tear in their realm, behind that lake house in the middle of nowhere Idaho, other Golden Tears existed elsewhere. One was in the land where the boys were never born. One was in a realm where Sam and Dean did exist, but grew up with Adam. In one universe John was a great dad, and they had the Apple Pie life with Mary at his side. In another John's the one who died that fateful November.
This story picks up when the Golden Tear in space and time appears inside the Cage in Hell, in the realm where we know Dean and Sam. Did you forget about the Cage? There were still two residents there, since the one everyone thinks about broke out wearing an Angel suit of our favorite Seraph of Tuesday.
Two beings were watching that Golden Tear warily.
One was human but barely resembled it anymore. His sandy blonde hair was once clipped short and neat. It now hung almost to his shoulders, in long, dark, dirty tresses. He needs a shave, a bath, new clothes, and other things. He's aged, just like everyone topside has. In Hell, time moves faster. He's got in gray mixed into those long dirty locks There are creases on his face, forehead, and tugging at the edges of his eyes. His gray beard is scraggly. Overall, he's incredibly weak. He's on the downside of middle age for this realm. He can barely stand up, he's so emaciated, as he squints. The Golden Tear is too bright for his clouding blue eyes. He must turn away a bit.
The other being, well, there aren't words in this realm to describe him. He outshines that Golden Tear. He recognizes it for what it is. He knows what it could mean. He turns to the human, and asks, one last time, if it's okay to share space.
Once consent is given, the two, now one, move towards that rip in space-time. Things turn so bright, upside down, inside out, and they ride the wave into the unknown ….