Set about 2 weeks after Journeys End. Meta-crisis doctor never happened. Donna and Rose both stayed with the Doctor. Just a bit of fluff.
It was quite simple really.
Once the battle was over, once they brought Jackie back to the parallel world, it was obvious what was going to happen next.
Because Rose couldn't stay there. Not when she had spent the last few years dedicated to finding him. Jackie knew Rose wouldn't come back. She was completely aware of Rose's feelings for the Doctor, and she had suspected he loved her back. But when she saw them united on the Crucible, both of them relived to have the other again, her suspicions had been conformed. So how could she not let her only daughter go? It was the only way she was going to be happy. So Rose stayed without a fight. And the Doctor wasn't complaining.
Before they were separated, they were kinda sorta together in a way. Though they never said as much, they were painfully aware of their own feelings for each other, and the fact that the other felt the same. However they thought they had more time. And the moment they admitted it to each other, they lost each other for what they thought would be forever.
Now that they were back together, it was almost like nothing had changed. (Almost. Because they hadn't been like this. Not this close. Not openly in love like this. ) But the routine had set back in by the second night, cuddling in the library with hot chocolate and a good book when the adventure was over. It would've set back in the first night, but they hadn't seen each other in years so I guess the sex was inevitable. Could you blame them? (Well, Donna could. At least jokingly at breakfast the next morning.) The only difference in the routine was they no longer went to separate rooms afterwards.
"I use to just stare at the stars and wonder if I'd ever see that little blue box again." Rose said one night, as she and the Doctor lay in bed staring at his starry celling.
"Is it selfish of me to be glad you never gave up on me?" the Doctor asked quietly.
Rose propped herself up to look into his eyes. "Maybe." She said, kissing him tenderly. "But I don't care."
"I didn't find you in the end. You found me." The Doctor said. Rose shrugged and rolled back onto her place in his arms. "I didn't think the universe would ever allow me to have you back." He whispered.
There was silence for a while. Both of them content to just lie there, relishing the feeling of being back together.
"Donna knew who I was." Rose realized. "We didn't have to explain anything to her. You talked about me."
"Guess I did. I was pretty mopey actually."
"You didn't mention anyone to me. Not even Sara-Jane."
"I didn't love them like I love you." the Doctor said simply.
He had said it before. He had said it many times the first night and throughout the week they had each other back. But Rose still couldn't get use to the feeling she got whenever he said what both of them thought he'd never get the chance to. Nevertheless Rose didn't have an answer to his statement, so the Doctor continued.
"I met Donna literally the moment I not-quite finished saying goodbye to you. She just appeared in the TARDIS. But we only had that one adventure. I met Martha, but she fancied me-"
"Figures." Rose interrupted. She had guessed as much when she met her.
The Doctor smiled. "Do you want to hear the story or not?" Rose stayed silent, and the Doctor continued.
"Martha fancied me, and obviously the fact that I didn't love her back and that I went on and on about you multiple times didn't help. So she left. I met Donna again, one of the first things she did was ask about you."
Rose smiled. Her and Donna got on quite well. It was clear that Rose had no reason to be jealous of her the moment they met in Donna's parallel world. Donna obviously loved the Doctor, but not like Rose did. It was nice to have a girlfriend aboard the TARDIS, the closest thing she had to that before was Jack or the TARDIS herself.
"And when she told me about meeting you in the parallel world, I was terrified because I knew that meant something bad was happening. But I'm selfish enough to have been ecstatic about seeing you again."
"I had to watch you die. In that parallel world, I was too late to save you and Donna wasn't there to. And then again with that Dalek, I thought I was going to loose you." Rose's voice was shaky, and the Doctor tightened his grip around her protectively. The last thing he wanted to do was make her cry.
He kissed her hair. "I'm here now though. You're here."
And it was only the beginning.