So, I'm surprised no one else has mentioned this possibility, even on the fanfiction sites. So I'll just throw it out there.
What if Lucifer/Sam decides not to snap Dean's neck like a twig. What if instead Lucifer/Sam decides to keep Dean around for his amusement, hell, maybe even keep both Past Dean and Future Dean so he can have a matched set to play with. If he breaks one, he has a spare lol. Anyway...
Whether or not Dean, either one, ever had a relationship with his broher before is optional. Obviously this RP would contain non-con, violence, and torture. How much depends on how sadistic (ok thats funny) Lucifer/Sam is and how much he wants to break his toys.
Whether or not the RP will contain Dean/Dean is also optional.
Since I really only like happy endings (eventually) in stories, I see a few things that might happen eventually. Somehow they, the Deans, manage to kill Lucifer/Sam and Past Dean is returned to his own time to try to change the future, but of course now him and Sam have to deal with what happened to Dean while he was in the future. Or maybe Castiel (past) gets wind of what happened to past Dean and Castiel finds Sam and together they go into the future to find him. End up finding both past and future Dean, and after Past Sam defeats Lucifer/Sam they take both past and future Dean back to the past with them, because Sam refuses to leave any version of his brother behind.
Maybe Sam can get past Dean and Castiel can get future Dean lol. The combinations/options there are endless.