I know you said that you like canon divergent, so here’s a canon divergent soulmate AU where when you meet your soulmate, you’re marked with their date of birth and death.
Cas insists that, as much as he loves Sam, that they can’t be soulmates. He’s missing the requisite soul. Part of him is saddened by the thought that there’s someone out there better for Sam than him, and if he meets them Cas will have to let him go.
He rolls up his sleeve to show Sam his unmarked arms — where the date shows up 99% of the time — to further prove his point, adding that Sam doesn’t have a date mark either.
Sam, however, is adamant. He knew that there was no way he could love anyone or anything in this universe more than he loves Cas. Appreciating the thought, Cas smiles and they drop the subject. They have each other now and that’s all that matters.
Months later, Sam is helping Cas preen his wings. It had been a few decades since it had been last done properly (which for an angel, wasn’t long at all) and Cas appreciated Sam’s eagerness to help.
Besides, Sam liked running his hands through Cas’s soft feathers and Cas seemed to get a decent amount of pleasure out of it as well. As Sam brushed through softly, something black underneath caught his eye.
Carefully brushing feathers aside, he found “05-02-1983” in small black numbers written on his wing. And right below it was “03-02-2061.”
Sam let out a soft gasp before exclaiming Cas’s name. Cas laughs at the revelation, admitting his mistake. His theory is that Sam doesn’t have a mark because Cas is older and will probably live longer than the human concept of time can comprehend. But that doesn’t mean he won’t be very thoroughly checked for a mark later.
