So, I already know that Donna is coming back. So psyched for that. Anyway, I interpret "The girl. She will fly." to be Donna. And I am actually thinking about the Doctor/Donna/TARDIS thing that Katie wrote last year. That would be frikking sweet. The whole "The Girl..She will fly" jsut sounds so much like the "She is returning." thing from last season about Rose. I dunno, though. It was kind of random in general. And Christina did fly. But there was no reason for PL (Psychic Lady) to say it like that. Thought what Donna would be doing on that planet I have no idea. AndI have no idea why PL would pick it up if she wasn't.
Other possibility: Jenny. She's around. She may be able to fly. She has more probability of being on that planet than Donna. Could work. But my vote is for Donna Noble. Because the think is, just because he erased her memories does not mean that he erased teh Doctor part of her brain. So she is going to be brilliant. And very curious. And probably more observant. And she is going to notice that she is smarter than she used to be, or better at putting things together or whatever, and she will probably wonder why. Or, if they want to be less interesting, she will honestly have no idea why they need her to save to world/the Doctor. Because she totally will.
He will knock four times. I already know that the Master is coming back. Not sure why he'd knock, but he is actually quite insane. Could be Jack Harkness. Could be Donna's grandad for that matter. Who I also know will be back.