However, this is something I did last year as a comment fic and realized a couple of weeks ago that I didn't put it here.
So, here it is. The prompt from mulder2000 was 'Dean knows this isn't his Cas but it's close enough for him'
If you can't be
Dean lay on the bed, staring at the ceiling. Another motel, another sleepless night. He still felt bad about leaving Lisa and Ben, but he didn't belong in that life. Unbidden, his thought turned to missed opportunities.
Dean frowned. The ceiling seemed to be rippling. He didn't have time to react before a body fell through the dark circle that flickered into and out of existance. A body that fell on him.
"Oof!" Dean gasped and stared at the dark hair in front of his face. The familair dark hair. "Cas?"
The head jerked up and he was caught in a bright blue gaze. "Dean!" Castiel slid up Dean's body and took his mouth in a hungry kiss.
Dean automatically responded for a moment, then jerked back. "Dude, what the hell?"
Castiel gripped his face and stared deeply into his eyes. He pulled away and got off the bed. Dean sat up. "Cas," he started, then stopped when Castiel lifted a hand and Dean found he could no longer make a sound. Castiel spread his arms wide and Dean could feel the power flowing in the room. The lights flashed, and for a moment Dean could see Castiel's wings, then the lights blew. Castiel seemed to glow for a moment, then his arms dropped.
"Damn them, damn them to perdition." He started pacing again.
There was enough light coming in through the thin curtins that Dean noticed something that he hadn't before. Castiel wasn't wearing his trench coat. He wasn't wearing Jimmy's rumpled suit either. He was dressed all in black. Dean watched the lithe body move, remembered the kiss, and took a deep breath. 'Down boy' he told himself. He cleared his throat. "Cas?"
Castiel turned to look at him. He waved a hand and the lights were fixed. He stared at Dean. "Where is he?"
"This dimensions version of me. Where is he, why isn't he here with you?"
Dean looked away. "Cas is back in heaven. He's in charge now."
Castiel looked shocked. "In charge?"
"Yeah, after we put Luci back in his cage, God gave Cas an upgrade. He's an archangel and he's in charge."
"But what about Michael?"
"Mike's in the cage with Luci, Gabriel's dead and the last we knew Ralph was still stuck in a ring of holy oil fire. He figured that God gave him the upgrade because he approved of Cas' helping us, so he went to be the new sheriff."
Castiel still looked stunned. "He left you to be in charge of those dicks in heaven? He's an idiot."
Dean frowned. "Wait a minute, you said this dimension. You're from another dimension? How did you get here, and how do you get back?"
Castiel growled. "I am here because Michael and Ralphael sent me here to keep me from being with Dean. And when I find a way home, they will pay." He started pacing again, but stopped after one turn of the room.
"You stopped Luicfer? The apocalypse is over?"
Castiel came and sat on the bed. "Until I find a way home, I want to stay with you. You can tell me all about how you defeated Lucifer and perhaps we can use the information in our battle." He leaned closer.
Dean leaned back. "Dude, personal space."
Castiel moved quickly, pinning Dean beneath him on the bed. "Dean," he growled softly, "I can read your mind. You want this. You want me, just the same as my Dean wanted me. We've been together for over a year."
Dean swallowed. "Yes, I want Castiel. But you're not him."
"I am him, just a him with slightly different experiences, and a lot more sense. I know a good thing when I see it, and I'm not afraid to say so."
"And one day you'll find a way back to your Dean, and I'll be alone again."
"Do you really want me to stop? Do you really want another lost chance. Yes, I will leave one day, but until then we can have each other." He leaned down and kissed Dean again, rocking his erection against Dean's thigh.
Dean remains unresponsive for a moment, then grabs the back of Castiel's neck deepening the kiss. Dean knows this isn't his Cas, but it's close enough for him.