*glares hard at RTD*
OK, I was going to make a sarcastic comment about how Rose better be evil or going to die, and then I thought....
Maybe (fingers really tightly crossed) Donna pushes Rose off the Medusa Cascade, thus saving the universe. The Doctor acknowledges same but will never speak to her again, explaining why River Song doesn't recognise her.
It would totally cement Donna as favourite nuWho companion :)