There could be a zillion ways to bring Donna back, and I hope they use one of them. She is part Time Lord, because otherwise taking away his mind/memories would have brought her totally back to normal. I love Donna. If she were going to leave, I wish she could have flown off to save planets the way Jenny got to. But the thing is, the Doctor can say that bringing her back is impossible, but considering how often he says "That's impossible!" and then fixes the problem ten minutes later, you'd think he could do something about it. Really, all he'd need to do is say "Brilliant!," talk really quickly about something involving the word "psychic," say "Oh!" a few times, and then tap Donna on the head. That's all I'd need.