This idea popped into my head as I finished the episode 'Damaged Goods' and the little bugger refused to leave me alone. So, here we are!
Hope y'all enjoy this little Supernatural 'What If'!

This is what happens when Sam Winchester puts his mind to an impossible problem.
He solves it.

Driving home in separate vehicles, going practically straight to their rooms after that, the brothers Winchester didn't have much time for talking after the whole 'this is the only way the entire world doesn't go down in flames' revelation back at the barn. But that was probably a good thing. Because Sam really wasn't sure what he would have said, or possibly done, if there had been.

Suffice it to say that Sam did not like Dean's little sarcophagus, permanent dip in the Pacific pond plan. In fact, he'd outright rejected it. On even a theoretical level.
Not to Dean's face though. Not right away. That would've been a terrible idea.

No, Sam was determined to show Dean just how ridiculous and unnecessary his suicidally bent plan really was, so once the both of them had had some time to calm down, the taller brother had confronted the older and proposed his own solution to their little Michael problem. In the form of a drawing.
Or, more like schematics that had taken him half the night to think up and the majority of the next day to commit to paper. Blueprint paper. Blueprint paper at which Dean had just shaken his head and chuckled.

"Well, Sammy, what will you and that egg head of yours think up next?"

"I-it'll work. I swear-uh, promise it will. It has to," Sam said, voice earnest as he smoothed the large rectangle with one giant palm. "And what's wrong with the shape of my head?"

"Oh, nothin'," Dean said, in a way that made it sound like he was withholding information. "But, uh, this," he said with a wave at the blue paper on the wooden tabletop, "crazy as it is, really does look like it could work. If a build like this was something folks like us could actually pull off."

"Maybe it wouldn't be possible for folks like you," suggested a voice that made the brothers jump, "but for folks like me?"

"Cas?" Sam heard himself ask.

"You could see that from all the way over there?" Sam heard Dean ask.

"Yes and yes. Respectively," the angel said with a nod to each hunter in turn. Before shutting and locking the bunker's main point of entry behind him. "I am indeed myself, and my eyesight is as good as ever." Sam raised an eyebrow and Dean gave a snort while Cas came down the stairs from the front door.
"I came as soon as I could, Dean. I had a little car trouble on the way over," the angel admitted with a small grimace.

At that, Sam got a very 'the hell, man?' look from Dean, which was honestly a lot better than the younger brother'd been expecting.
When Cas came to stand before them though, Dean addressed the angel as if he wasn't totally peeved. Which was way better than Sam had been expecting. Honestly.

"Well, good to have you home, Cas, but what's this about 'us folks' and 'me folks'?" Dean asked, brow furrowed.

"I simply meant that, with the assistance of my admittedly rather limited divine grace and my fluency in Enochian, Sam's well thought out, beautifully inscribed idea, could very well be brought to reality."

"Come again?" Dean asked with a tilt of his head.

"You think my sketch is beautiful?" Sam couldn't stop himself from blurting.

"Of course. The senses of depth and perspective are nothing short of art," Cas said as if it was a simple matter of fact. To which, the artist just about blushed.

"That's a sketch?" To that, Sam gave Dean a long suffering look as Cas came between them to inspect the blueprint properly.

"Yes, it would surprise me more if it turned out we failed to finish this successfully than not," the shortest out of the three said. The finger of an outstretched hand tracing one of the numerous sigil along the sketch's borders. "After all, Dean already has the instructions and the first hand knowledge, having already successfully constructed a sarcophagus of this basic design. In secret," the accusation coming out only as biting as it deserved to be. "An entire, generously sized, free standing cage should not be so different. If one stretches their imagination. A little."

"Wow, listen to Mr. Doom And Gloom over here," Dean said, looking over Cas's head to catch his baby brother's reaction.

Sam, not giving Dean the satisfaction, kept his eyes front and-

"I don't understand; I'm not being negative at all," informed a Cas who's face had taken a turn towards full blown confusion. "In fact, I'm pretty sure I'm being encouraging."

"Uh, yeah, Cas, we know," Sam huffed with a reassuring smile. "But, a cage wasn't what I had in mind, per se, while I was designing this thing. More along the lines of... a room, maybe? Somewhere Dean could visit when needed, or maybe live, if it came to that," he said, giving said brother a sidelong looking over. Trying to gauge his reaction.

"You want to give Michael a man cave?" Dean accused.

"No, I want to keep you out of the Marianas Trench. And alive and with us. And I think this is the way to make that happen. Even if only until we find a more permanent solution. And besides, you already said it looked like it could work," Sam said. Trying, with marginal success, to reign in his reflexive, younger brother, 'so there' tone.

"Is that so? Well, in that case, what say you, Dean? Are you willing to give this- uh, this Room a shot?" The angel asked, turning to the hunter in question.
When there was hesitation, Sam watched as Cas's expression softened that last bit and the angel put a hand on Dean's shoulder. Approximately where he had those so many years ago, and where the proof of that encounter had once stood for the world to see. Angry and pink and righteous. "The world is a better place with you in it, I assure you."

"Thanks, Cas. You're not so bad yourself," Dean said, making no attempt to shake off the angelic touch. Looking, to Sam anyway, like he might be hiding a blush.

"Alright then," Sam started with a clap, "it's settled. We start work in the morning. Dean, you're making breakfast." Sam chuckled when the statement got him two surprised faces. "We're gonna need it."

"If you don't mind Sam, I'd like to study the schematic further and potentially make a few Enochian suggestions for the warding."

"Yeah, please," the hobby-architect said, motioning for Cas to 'have at it'. "And I'll look up where we can get our hands on that much fallout shelter grade metal."

"Good luck with that," Dean said with a note of derision. "It was hard enough finding as much as I did. Besides, it needs to be consecrated in fire and-"

"Oh yeah, Cas, the back has the plans for the interior," Sam said. Cutting off Dean as if the man haven't even spoken.

"Excellent. I hadn't given that a thought," said Cas as he turned the paper over. "Ah, that's right. We'll have to build the floor as well."

"Right?" Agreed the architect in the room. "Because it basically has to be a big box."

"A big box to hold a very bad angel."

"Bad enough that those sigils need to be permanent. I'm thinking, to make them smite proof: we weld them."

"Amazing, Sam. You're attention to detail is commendable," Cas said, tone a little more than impressed.

"Seriously? What am I, chopped liver?" Deena said with a scowl tilting toward petulant.

"No, Dean. You're the guy we're going to save. And we're not gonna give up on you, no matter how badly you might want us to," Sam said with a victorious smirk.

"Yes, Dean, there will be no giving up. In this, we are together until the mission is completed. Successfully," Cas tacked on. A small smile gracing his face.

"I- I don't want anyone giving up anything. It's just... What if we're setting ourselves up for failure? Huh? What if we somehow build that crazy thing and- and we lull ourselves into a false sense of security and the world fries anyway?" The guy with an archangel locked away in his head asked, motioning towards the blueprint halfway through.

"Dean," Sam said as he took a step around their resident non-arch angel, closing the distance between brothers, "my idea's not 'crazy'. What's crazy is what you were planning to do, okay?"

"He's right you know," Cas noted.

"And the world isn't gonna fry, man. I mean, come on, when have we ever not been able to stop the apocalypse?" Dean's eyes grew in the face of Sam's sheer logic.

"Apocalypse World," Cas said. Inadvertently bringing all eyes to himself. "Perhaps we can forget that one."

"No, that's a good point, Cas," Sam said with an earnest nod. " The only world we know of where we didn't stop the Apocalypse is one where we were never born. If that doesn't make for good odds, then I don't know what does."

"Well, sounds like now my plan's Plan B," Dean said with a rather bemused tilt to his eyebrows. Before giving his hands a firm clap. "So, what do you two chuckleheads want for breakfast?"

At that, the entire room smiled.

Yep. Not gonna happen on the show, especially not with ep. 11 and all, but I thought I'd throw my two cents out there!
Feel free to lemme know what you think so far and please take comfort in the knowledge that Chapter two should be right around the corner! ;D