Not to mention "Father Christmas isn't real." "Isn't he? Red bike when you were ten?" What if he missed her so much already that he visited her through her timeline, just to keep tabs? Maybe even to figure out the best way for her to say "yes.":
Doctor: I know. When I look at her now I can see them. The hole they left, all the pain that filled it. I just don't think I can face that every day..// Donna: It won't stay like that. She'll help you. We both will..// Doctor: When they died that part of me died with them. It'll never come back. Not now..// Donna: I'll tell you something, Doctor. Something that I've never told you before: I think you're wrong.