Based on this post by deansass on Tumblr:
Theory on how Supernatural should end: Dean Winchester wakes up and he's frightened and sobbing, so Mary Winchester turns on the lights and brings her 4 year old son into her arms and tells him "it's okay, Dean, just a nightmare. You can sleep, angels are watching over you" and he wants to be happy but then he sees Castiel standing at the corner of the room smiling softly.
Dean's scream of, "Mommy!" could be heard through the whole house. John sat up and looked at his wife. "Mary, do you want me to get him?"
She shook her head and slipped out of bed. "No, no, I've got him." Allowing her husband to go back to sleep, she took quiet steps down the hall to her eldest son's room. Scooping the frantic four year old into her arms, she asked, "Sweetie, what's wrong? Was it a nightmare?" Dean wrapped his tiny fists in her nightgown and wailed again. Carrying him with her, she flipped on his light. "Look, honey, look. No monsters. It's okay, Dean, just a nightmare."
Dean's lip quivered again, but his tears stopped. He kissed his mother sloppily on the cheek, as only a four year old can, and released her nightgown so she could lay him back down. "You can sleep, now. The angels are watching over you," she murmured, kissing him on his forehead. Her sons eyes were locked on something she couldn't see in the corner. "There aren't any monsters here, baby, I promise."
Dean's eyes flicked to her. "Not a monster, mommy. It's okay now, my angel is here."
She smiled at her son's vivid imagination. "That's right, sweetie. Your angels are always with you." She tucked him back in and returned to bed.
TWENTY YEARS LATER
Dean Winchester's eyes were practiced in the art of finding people. Usually his parents and kid brother. Or his roomate, Charlie. But he'd already hugged his parents and he'd given Sammy his obligated knuckle sandwich, and he and Charlie had already tackled each other and pushed their degrees in each other's faces while shouted, "GRADUATION, BITCHES!" (And in the process, terrifying the cloister of visiting grandparents to their right.)
No. He was looking for another face. One that was just as familiar to him as his family's and his best friend's faces. One that hadn't aged a day in all of Dean's childhood. "Cas, where are you?" Dean muttered, scanning the crowd. "You promised you'd be here today."
"And so I am." There was a fluttering sound, like wings, and when Dean turned the man was there. He'd watched him through childhood. Been a teacher when Dean struggled through pre-algebra in middle school. Had comforted him through breakups in high school. And now was finally here, smiling at Dean with pride, his blue eyes shining in the way he had that was not tearful, but so happy that they just seemed to shine. "Congratulations, Dean Winchester."
"Cas." Dean pulled the man in the trench coat into his arms and hugged him with all his might.
Castiel's hands were hesitant, as they always seemed to be when Dean touched him, but all the same he slowly came to pat Dean on the back. "Dean."
Dean pulled back, his eyes suddenly quite serious. "So does this mean you're gone, now?"
Castiel frowned, looking confused. "I don't understand your question."
"You said you had to make sure I grew up right. Well, I grew up. So does this mean you're taking off?" The young man's mouth was set in a thin, grim line.
Castiel tilted his head to the side, studying his strange companion. Most of the time, this Dean was how he should have been. Light and happy and hopeful. But sometimes Castiel saw something different. He had done all he could to make sure that time was righted and the Winchesters' lives were never forced down the path that they had originally led. Yet he could not entirely rewrite his friends' existences. The truth of what had been still clung to his mortal friends, and he could see those hardened men shining through their younger forms at the most seemingly random of times. "Dean Winchester, I have no intention of going anywhere. You and I share a most profound bond. That bond does not break because you get this…master's degree."
Dean's face split into a wide grin and he threw his arm over Castiel's shoulder. "Come on. I want you to meet some people."
"That is not a wise decision, Dean," Castiel began. But it didn't matter. Dean was already tugging him toward the crowd of people who were waiting for him.
"Guys, this is Castiel. One of my best friends."
Charlie pouted. "I thought I was your best friend."
Dean rolled his eyes. "Charlie, this isn't Highlander. It's not all 'there can be only one.' Okay?"
Charlie's eyes narrowed. "Wait, Castiel? Like, the guy you're in love with?"
Dean turned red. Sam snorted and began to laugh. John just smiled at his son. Mary frowned, having remembered hearing stories about a 'Castiel' that watched over Dean when the boy was a child. Castiel nodded gravely to her as Dean reached back to grab his hand. She simply smiled and reached her hand out. "It's good to meet you, Castiel."
"Thank you, Mrs. Winchester. In my personal opinion, these are the best circumstances under which we could have met, and I am honored to be part of it."
Mary frowned. "What circumstances are those?"
Castiel merely smiled. "Dean is happy."