Okay, okay, okay, but I CANNOT get this AU idea out of my head:
Castiel, as a Reaper instead of an Angel.
Castiel, meeting Dean for the first time when Dean is four years old, standing in front of his burning home with the flames reflecting in the tear tracks on his face. Castiel revealing himself to Dean, gently prompting Dean to mind Sammy’s head as the infant cries and squirms, because he really doesn’t want to reap more than one soul tonight. Laying a sorrowful, sheltering hand on Dean’s head and staring down into pleading green eyes and whispering, “I’m sorry,” before walking slowly up the burning porch to reap Mary Winchester’s soul (who refuses to go with him anyways).
Castiel, as the Reaper who appears to Dean after the car accident while Dean is in a coma, shocked when Dean remembers him from that night so many years ago. Dean jokingly asking, “Are my guardian angel or something?” and Castiel sadly telling him, “No. Rather the opposite, I’m afraid,” and having to explain to Dean about the existence of Reapers. Castiel wishing to himself that he didn’t have to reap this vibrant young man, who is brave and frustrating and stubborn and obviously so full of life, and then realizing he should be careful about what he wishes for, because Dean is spared but Castiel is forced to reap Dean’s father instead, and he hates that he’s causing Dean more pain.
Castiel, meeting Dean again less than a year later. And then meeting him again. And again. And again, as Dean and Sam work the Trickster case, and Dean dies every day. They get to know each other pretty well, and it becomes something of a running joke: “We’ve got to stop meeting this way, Cas,” Dean teases, and each day Dean bemoans the ridiculous new way that he’s just been killed, and Castiel commiserates sympathetically and helpfully points out that at least Dean didn’t pee himself this time. And he hates that Dean has to die every day, but he hates himself even more because he can’t help dreading the day they catch the Trickster and it all stops, because then he won’t have an excuse to keep seeing Dean and listen to his laugh and hear about his favorite bands and watch the fond way he looks at his brother.
Castiel, appearing when Sam is stabbed by Jake, sick to death with the thought of reaping the soul from Dean’s brother. He doesn’t reveal himself to Dean in the real world this time, even though he could, because he can’t bear to have Dean turn those stricken green eyes on him, he’s a coward, a coward, a coward - but it doesn’t stop Dean, kneeling in the dirt and clutching the lifeless body of his brother tight in his arms, from screaming out, “I know you’re out there, Cas! Don’t you dare take him! I’ll never forgive you!” And he thinks it’s a cruel, cruel joke that he’s destined to continually reap the souls of the people Dean loves most, one by one, and when Castiel leads away the soul of Sam Winchester, there are tears on both their faces as they tell Dean ‘goodbye’, even though he can’t hear them.
Castiel, being summoned one year later, unsure of what’s happening, suddenly finding himself staring down at Dean’s shredded body on the floor at his feet - but Dean’s soul is still here, obstinate and unyielding, circled by snapping hellhounds but refusing to let them drag him away because “I said I’d go to Hell and I will, but I don’t need hand-fucking-delivered by these fleabags, I’ll take my own way there, goddammit! I’m allowed a Reaper! Bring Castiel the Reaper!” And Castiel raises his eyes and meets Dean’s gaze, and it’s gentle, and resigned, and frightened, and forgiving, and Castiel doesn’t deserve it, he’s never deserved anything less than the understanding in those eyes, and he’d rather be anywhere else, he’d rather be dead himself, than here to take away Dean’s soul to Hell. But he does his job and he leads Dean to the gates of Hell, except then he can’t go, he can’t leave Dean here, he can’t - until Dean kisses him, sudden and fleeting, and tells him, “It’s okay, Cas,” and pushes him away.
Castiel, blindly turning away for only the briefest span of time - the blink of an eye, the pulse of a human heartbeat - before he realizes he can’t do this. Screw the job, screw the deals, and screw the laws of nature, he will not leave Dean here…except when he turns around, Dean is already gone.
Castiel, spending the next forty years breaking into Hell, laying waste to horde after horde of demons with the fatal touch of his ghastly true form. The memory of Dean’s kiss burns a brand against his lips, and when he finally, finally finds Dean, the touch of Castiel’s spectral hand burns its own brand on Dean’s soul as he grips him tight - the touch of death claiming a soul already dead, because Dean is his. Dean looking up at Castiel, and his soul is messy and tortured and broken, but he still manages a smile as he chokes out: “See? Told ya you were my guardian angel,” and Castiel carries Dean’s soul out of Hell and chooses life.