Thinking half-thoughts but like... what if, in order to pull Cas out of the Empty, Jack had to leave the vessel behind? I've seen theories on getting Cas out by leaving the grace behind (which does make more sense lore-wise) but stick with me...
They can't get the vessel back or create a new one whilst it still exists (whatever, the logistics aren't the point) and obviously Claire is the only other bloodline vessel which isn't even worth entertaining. So essentially Cas is stuck in Heaven in his true form. And in the beginning, he did try and check in on Dean but it hurt too much to see him so listless and spiralling and not being able to do anything about it so he just went cold turkey and threw himself into helping Jack rebuild Heaven.
Until he feels a barrage of emotions so strongly that it would have brought him to his knees were he to still have any. Pain, regret, sadness, acceptance, hope... a cacophony of chaos and he knows the source immediately. And he knows the reason. Dean is dying. It's barely been the blink of his many eyes and Dean's already dying. And there's nothing he can do about it.
But he could at least be there for him, even if Dean can't see him or know he's there. So he flies down to some decrepit barn to find Dean and Sam. Immediately, he is overcome with the need to FIX-IT. Why should he accept this? Why are any of them just accepting this?? If only he could...
And then an awareness shakes him to his core. The vessel is willing. The vessel has given permission. And Cas doesn't give himself time to talk himself out of it. He'll beg forgiveness later, just as long as Dean is alive.
And so he possesses him. Sam's still cradling his face and crying when Cas speaks through Dean's voice. "Pull him down."
Sam sniffs. Blinks. Frowns. It takes him longer than it usually would to connect the dots. Too long. "Sam!"
Sam starts and makes a grab for a weapon he doesn't have. "Who are you?"
"It's me," Cas says, also not thinking too straight through his own panic and the sudden onslaught of Dean's emotions battering him from the inside. "I can't heal him with the rebar still in. Hurry!"
Sam isn't hurrying. "Cas?"
In a display of trust that Cas will be grateful for later, Sam finally bursts into action, pulling Dean from the beam, marvelling at how Cas keeps him upright. Then he begins to heal him from the inside, pouring his renewed grace into the wound and the rest of his body just because why not when he's already there?
Blinking Dean's eyes open, he finds Sam waiting, anxious. A nod from Cas has Sam sucking in a breath and launching himself forward to hug Dean. Or Cas. Or both.
It's nice. He wishes he could stay but he's done what he needed to and it was time to leave them to it. Shrugging out of Sam's grip, he offers a sad smile as he says, "I'll be waiting for you both. Just take your time about it, please."
It's clear Sam wants to argue but he needs to leave, now. And so he does.
With a frown, he tries again. But still he remains. And Sam is now arguing but Cas can't focus, he's too busy panicking. And Dean is hammering on the little door in his mind that Cas put up to dull the unpleasant feeling of being possessed and Cas tries sending him reassuring pulses that yes, he's trying, he'll be out soon. But strangely that just increases the pounding which take on an edge of desperation until Cas has no choice but to open the door and-
"DON'T YOU FUCKING LEAVE ME AGAIN."
Everything stops. The pounding in his head, the tether on his grace, the desire to flee. The only thing that remains is an overwhelming sense of anticipation. And Sam still rambling about something that is probably very heartfelt and that Cas absolutely could not give a shit about right now.
To test a theory, Cas tries again to exit the vessel, only to have what he now realises is Dean's soul clamp down on his grace, keeping it rooted, nestled inside him.
"You ain't going anywhere, sunshine," comes Dean's shaky voice from inside his head. "We got some shit to talk about, you and me."
And so talk they do. And when Cas says that he's without a vessel and that's why he hasn't been by, Dean tells him that now he does. Just like that. As if he hasn't spent the entire time Cas has known him trying to avoid being a meat suit for an angel. But Cas can hear the eye roll as Dean says that Cas has been the exception to that rule for awhile. He made his peace with that fact years ago. Which explained the open permission he seemed to have.
"So, listen. We'll try to figure out how to get your body back. But, if we can't? Don't be using that as an excuse to not visit, yeah?"
If Cas had the choice, he'd never leave.
A flash of warmth roll through him, reminding Cas that he's not alone with his thoughts right now.
"Well. That'd be OK with me, too."
Cas smiles with Dean's lips. But that's OK because Dean's smiling too.
"...Have you listened to anything I just said?" Sam asks.
