The End has spawned a rich tradition of fanfics, AUs of a canon AU. Not all of these are End!verse fics. Some of them explore what would have happened if Dean's glimpse of End!verse hadn't occurred, if Sam and Dean had stayed separated. And some of them look at how the End!verse itself might have gone differently.
For the prompt: In the months before he says 'yes' to Lucifer, Sam keeps in touch with Dean. He doesn't call, because he's too afraid that no one will answer, but he sends letters. Sometimes it's just a postcard with a few words, but he writes faithfully, every week. Until one day he doesn't.
A bleak fic, as painful for what we and Dean don't find out about Sam's life leading up to his Yes to Lucifer as for what we do learn.
Under mysterious circumstances bitterly disputed by the historians, Sam Winchester said yes to Lucifer. Three years later, Lucifer is gone and the familiar landscape that the Winchesters once traversed is a burnt-out ruin of its former self - not to mention the damage done to Lucifer's vessel. Trapped in a grotesque parody of domestic bliss with a brother he no longer recognizes, a shell-shocked Dean has no reason to believe that happy endings exist for men like the Winchesters.
A classic, and hopeful in its way, this explores what the aftermath might have been if Sam had survived after his End!verse envesseling of Lucifer. I find this too painful to reread (I have particular issues with memory), but it's beautifully done.
For the prompt: Dean, Dean - can even be Dean/Dean if you like - another Five Years prompt. Past Dean catches a stray bullet close to Christmas, the wound gets infected, and Past!Deano gets increasingly feverish and frail. As it happens, all he wants for Xmas is his brother back, which he can't have. Future!Dean only has himself to give ... and one of Sam's hoodies that he's very carefully (and likely secretively) carried around with him for the last few years. Future Dean is perhaps uncomfortable with the sentimentality of it all, Christmas is no doubt pointless at this stage of things, and all of that, but ... what the heck.
Everyone should read this author's After The End 'verse as well (another Sam survives Lucifer fic, with more-bitter-than-sweet Sam/Cas), but, since that one has been recced here quite recently, I'm pointing you towards one that is possibly even more painful. This is hurt without the comfort, movingly written.
AU of the 5.04 universe (in which Sam is, and has always been, at Camp Chautauqua). It’s funny, the ways in which Castiel has learned to experience regret.
A somewhat more cheerful variant on End!verse, with awesome Cas characterization.