This chapter takes place during "The Runaway Bride" in the original (or non-"Turn Left") universe. It's not quite as fluffy as the first chapter, but I think it fits rather well.
Chapter 2, Star-Cross'd
The memories of his Christmas with Rose played through the Doctor's mind, even as he watched the Empress of the Racnoss scream for her children.
"Promise me you won't be alone. That whatever happens, if I'm ever... that you'll find someone."
"I'm sorry, Rose," he said quietly, his words swallowed up in the chaos surrounding him.
Another promise broken. He simply wasn't worth the faith she'd placed in him. He'd expected to die during the Time War. His people, his world had burned, but he hadn't been worthy to share their fate.
Perhaps he'd been spared to meet just this end. He would drown, fulfilling yet another of the Time Lords' genocides.
He was almost glad that Rose was gone. She's safe with her family. He repeated the words to himself, ignoring the tears streaking down his face.
She was safe.
And he was alone. As he should be.
Donna's voice startled him, and he glanced down, amazed to see the would-be bride standing amid all the destruction he had caused.
"You can stop now!" she shouted.
He looked back to the Empress. The Racnoss' screams were calling to him, calling the last of the Time Lords to give in and let fate consume him.
But so was Donna. This incredible, abrasive woman who had so impossibly come into his life... just when he'd needed it. Could it be that Rose had somehow sent Donna to him? She had once held the Vortex within her, able to control time and space.
"I want you safe. My Doctor."
Maybe he wasn't alone, after all.
"Come on!" he called to Donna, snapped out of his reverie at last. "Time I got you out!"
He turned his back on the Racnoss, and headed for the stairs.
Even now, a universe apart, it seemed Rose was still saving him from himself.
If she had done this for him, the least he could do was to accept her gift.
After all, he thought as he led Donna out of the tunnels, a promise is a promise.