What if Dean had killed Sam at the end of season 10?

Death had kept his promise. He had put Dean somewhere where only plants and microorganisms lived. Nothing he could kill, not really. Nothing that would satisfy the Mark of Cain.

When the Mark was too much he would kill the vegetation. Miles of small plants ripped up by their roots and hacked, slashed, and ripped apart.

Death would visit sometimes. Always with some junk food, usually including burgers for Dean.

And sometimes when the Mark overwhelmed Dean, he would bring company. Usually a mass murderer, or a monster. And he would sit back while eating his food and watch the Hunt. Not that it ever lasted long. The Mark was too hungry for that.

Sometimes the Mark of Cain would retreat and go into a type of Stasis and then and only then would Death bring Castiel.

The three of them would talk for days. Reminiscing about their family and friends. Catching up on the news and just enjoying each other's company.

But it was never long. The Mark would wake up, and Castiel would leave again, waiting sometimes centuries until the Mark went into Stasis again.

Dean hated being alone. So when the loneliness took over he would pull out the pictures of his family that Sam had given him in his last moments and mourn what he had lost.