I wouldn't call this an AU, but the timelines are a bit strange. It's Sam from Season 4 with Cas and Dean from the end of Season 9.
They were an unlikely couple. But Dean supposed there was something to be said about the saying "opposites attract". Not that they were really opposites anyway. They had both broken away from something they thought was wrong, and they had both done it for someone they loved.
The first one was an angel, Castiel. The fallen angel. His trench coat and backwards tie and bright blue eyes. His tousled hair and deep gravely voice. He looked pretty unassuming until you saw his wings. And Dean had seen them. They were huge and as black as night. His wings were proof of his angelic grace. But now they were gone, and Castiel was a just a multidimensional wavelength of celestial intent stuck in a dying vessel. Cas was the man with the stolen grace.
The other one was also broken. The human, Sam. His soft smile belying the hazel eyes clouded with pain. He had rebelled much like Cas. From his father, from hunting, from everything his father believed in. It hadn't worked and when he went back to hunting he wasn't a normal human. He had drunken the blood of his worst enemy to awaken his powers. Dean had hated those powers, even if they were of some use to kill demons. Sammy was the boy with the demon blood.
But they could be together now. They didn't have to save a crumbling world. There was nothing good left anyway. They could just run away like they had tried to do before. Dean wasn't there to stop them. Dean was dead and gone, slain by an angel Cas and Sam hadn't been able to stop.