Disclaimer: I don't own any part of Doctor Who, unfortunately.
Author's Note: Because sometimes an idea takes hold and won't let go no matter how improbable it is.
The Doctor and River fell through the TARDIS doors, gasping for air, weak and beaten. It shouldn't have ended like that. It should have been easy. But he was never at his best around her; always trying to show off or protect her. He was bad about letting his emotions get in the way and nothing was stronger than the love he had for her. When the Doctor saw River get hurt, he lost control. An emotional Doctor makes mistakes. And he made enough to cost them everything.
"River, I am… so… sorry," he said, shaking his head as they helped each other to their feet.
"Shhh, it's okay, we'll be fine." She cupped his cheek in her hand, wiping away a tear with her thumb.
"No, River… It's too late… I'm regenerating." The Doctor held up his hand for her to see the subtle golden glimmer spreading across his skin.
"So am I, my love." She lifted her hand and pressed her palm against his, the luminescence mirrored in her flesh. He gazed at the light in astonishment and wasn't sure if it made him feel better or worse.
"Just think," River said tenderly, "We'll be able to discover each other all over again. We'll be new together. We'll actually share firsts." The Doctor managed a sad smile at her words.
They came together in a passionate kiss as the radiance grew, entwining their embracing bodies. The glowing tendrils from River mingled with his, twisting and curving together in a sultry dance while still remaining distinct. The Doctor desperately ran his fingers through her hair, afraid he'd never again touch those wild, wonderful curls. Golden wisps flowed from his hands down River's body, lovingly caressing her curves. As the glow threatened to consume them, their minds shared only one thought; would they still love each other?
Their heads and arms flew away from each other, leaving only their bodies pressed together as their regenerations reached their climax. As their dying hearts were reborn, they knew. Each would love the other. Always.