Let's change "The End of Time" in the following way: While the Doctor is talking to Wilf in the cafe, Wilf asks him to come back and fix Donna, to give her back her memories and her life with the Doctor. We know that the Doctor can't do that, but let's change it so that at that moment, he realizes that he knows how to do it - he can use his current biodata to filter from Donna what's actually him and remove it, leaving just her behind as a perfectly normal human, with intact memories of her life up through traveling with the Doctor.
With that knowledge, he and Wilf go off to find Donna to fix her, but the other events of the episode begin to happen, and they proceed as we saw on screen. After the Doctor defeats Rassilon and Gallifrey retreats back into the Time War, Wilf knocks four times. The Doctor then must choose: if he saves Wilf, he not only dies, but the regeneration rewrites his biodata and he can no longer fix Donna; but to save Donna, he must let Wilf die.
So, the question is, what do you think the Doctor would choose? And also, what would you want him to choose, since they are not necessarily the same thing? (Note: Meta is not within reach, so you can't choose "Save Wilf, then go get Meta to fix Donna." ;) )