This is my take on what could have happened if Dean had stayed in his dream during What Is And What Should Never Be. Dunno how long I'll make it but I'll try to post two chapters at a time, one for Dean and one for Sam.
Dean stood in the dreary old warehouse, staring at the young girl strung up before him. This girl had been haunting him for the last few days, ever since he was taken by the Djinn. She looked very near death, pale, hollowed eyes with dark bags beneath them, face slack with no expression, gaze blank.
"Dean, please." He heard Sam say from behind him.
"What if I'm like her?" Dean said, not specifically talking to Sam, but more like talking to himself. "What if I'm tied up in here someplace? What if all this is in my head?"
Dean walked closer to the girl, continuing his musings. "Maybe it gives us some sort of supernatural acid, and then just feeds on us slow." He could hear Sam behind him talking but barely registered the words. "What if that's why she keeps appearing to me, she's not a spirit, but more and more I'm catching flashes of reality? Ya know, like I'm in here somewhere and I'm catatonic and I'm taking all this stuff in but I can't snap out of it."
"Yeah, ok, look, you're right, I was wrong" Sam said, pulling Dean away from the girl and back towards the entrance. "We need to get out of here fast."
Suddenly Dean whipped his arm out of Sam's grip, brushing his non-hunter trained brother off like he was nothing. Sam gave him an incredulous look.
"I don't think you're real." Dean said, slowly gaining his wits back.
"Dean, do you feel that?" Sam asked, grabbing his arms and shaking him. "Feel this, I'm real, this is not an acid trip! I'm real and that this is going to come down here and kill us for real."
"There's only one way to be sure." Dean said, face expressionless as he pulled the silver knife from his jacket. Sam backed away, holding out his hands in self-defense. Had the situation been different, Dean would have smiled, laughed and told Sam not to worry, that he would never hurt him. But he knows this isn't Sam. "Old wives tale, if you're about to die in a dream, you wake up."
Dean could hear not-Sam babbling on about how was all crazy, saw him step forward and reach for the knife. Dean held the knife out, almost threatening Sam with it, before again turning it towards himself, reassuring himself that he'll wake up, reassuring himself that he's sure about this plan, even if not 100%. The only think that stopped him is the change in not-Sam's voice, as he goes from whiny and babbling to forceful in his command to stop.
As Dean looked up and glanced around, he could see his mother striding towards him from the right, looking just as she did the last time he saw her when he was four years old. Jess on his left, and his girlfriend behind not-Sam.
"Why'd you have to keep digging?" Not-Sam asked, now so calm. "Why couldn't you have left well enough alone? You were happy."
Dean steels his expression as his mom walks toward him and they stand face to face. She tells him to put the knife down, but Dean is too bewildered by everything around him that he can't respond. When he can finally speak, he can only tell her and himself that she's not real, that nothing in this dream world is real. She plainly tells him that it's better than anything he had outside of this world.
At first, Dean is taken aback by the comment. But as he thinks about it, he realizes it's true. He may not have dad, or hunting, and his relationship with Sam could use some work. But compared to the life he's been living since he was a child, this really is better. He listens as his mom tells him that they can be a family again. Afraid it will end too soon, he reminds her that he's a victim of the Djinn and he'll die in only a few days. She soothes him and tells him it will feel like years with his family, years in quiet, calm safety.
Jess reassures him that he'll get to see Sam live a regular life. Carmen tells him that she loves him and asks him to stay and have a family with her. It's Sam's words that hit him the hardest, that really convince him to stay in this artificial life his mind built.
"Why is it our job to save everyone?" Sam asks, "Haven't we done enough?" Dean hears Sam ask for the knife again, he hesitates for a second, unsure whether he wants to leave real-Sam alone and unprotected in the real world. His own mind reminds him that Sam can take care of himself, that he's said so several times and did fine on his own while he was in college.
Dean sighs, the weight of being a hunter lifted off his shoulders as he willingly turns over the knife and is immediately rewarded with a big, warm hug from his brother. His mother stands at their side, her hands on their shoulders, smile beaming. Jess and Carmen are also smiling as the setting around him morphs to his childhood home. Dean himself smiles and takes a slow deep breath, he's finally home.