Look, kids, I wrote a Superwho ficlet!
“Your death is a fixed moment in time,” the Doctor said, after Castiel finished vomiting lake water and the occasional small fish onto the deck of the TARDIS. “Can’t be changed! Must happen!”
Castiel wiped his mouth with the back of his hand. His mouth. His hand. His again. His grace was weak and sputtering, but it was there, and it was his, and he could move and breathe in this body. The black was gone, that roiling tide of inky slickness that had choked him down until he was sure he was dead; it was gone in search of better bodies to corrupt and play with, ones that weren’t blistered and sagging at the seams and half-dead already.
But still. His. Not even Jimmy’s, not anymore. Castiel’s heart and skin and nerves and grace and glow. His.
“I am alive,” he said, in that low-rumble hoarse voice. “Doctor. You saved me. Why?”
“Why not?”
“You said I must die.”
“Well, yes, obviously.” The Doctor bounced to the console and started pulling on levers, wacking things occasionally, checking his gauges.
Castiel stood, shakily. He was thoroughly wet, but somehow lighter. The trench coat, he realized, with a strange pang of sadness. He straightened his back. “Then I must.”
“Must what?” The Doctor looked preoccupied, frowning at some sensor.
“Die.”
“What on earth would you want to go and do that for?” The Doctor said, focusing his ancient gaze on the sopping wet angel. “You just went and got your life back, and you haven’t even seen Tirinian Three yet. Best burgers in the galaxy, you’ll love it.”
“Enough, Doctor,” Castiel said gravely. “If my death is fixed, then I must die. I will not risk this world again. I’ve done too much already.”
“Oh, you poor foolish angel,” the Doctor said, crossing lightly to him. “Older than me and you still can’t wrap your head around the simplest things.”
Castiel stared at him. The Doctor waved his hands.
“You died, Cas! You’re dead and gone. All that’s left of you is a trench coat floating in the lake. You had your death. Now you get to enjoy your life.”
“I am not dead,” Castiel said slowly. The Doctor beamed.
“To the world you are. That’s good enough for me!” When Castiel stared at him stoically the Doctor sighed. “C’mon, Cas. Stay a while. Think of it: The two of us, dead and loving it! We could save the world. We could save many worlds.”
“The Leviathans…”
The Doctor put his hand on Castiel’s shoulder, suddenly somber. “I’m sorry, Cas, but you can’t do anything about them. You can’t help Sam and Dean, not this time, or time will stop. You’re dead. You have to stay dead. Come with me.”
Castiel paused. In the back of his mind he could still feel Sam and Dean. Scared, in pain, worried. Grieving for him. They had a hard path in front of them, but then again, didn’t they always.
Slowly, he nodded. The Doctor clapped him on the shoulder.
“Excellent! The angel and the Time Lord, oooh, I’m gonna like this. I might just keep you, you raggedy angel, you.”
Castiel watched him return to the console. It had been a long time since he’d stood in this place. He liked it. It was so expansive, like it never ended, spreading tendrils through time and dimensions, everywhere and nowhere all at once. The light was warm and soothing, the TARDIS murmuring in his ear, something that sounded like love, and welcome. The Doctor watched him with those bright, old eyes. “Where to?”
The Doctor pulled a lever, and they vanished, into the cosmos.