Out of all of the companions from the modern series, it seems that she collude return the easiest. She's still in the Doctor's universe, and her brain won't burn up if she thinks of him.
As far as I am aware, Steven Moffat promises an 'explosive' and 'special' 50th anniversary. They may throw a big party or something along those tracks. I think we can safely say Chris Eccleston won't be attending. Furthermore, in a convention, Tom Baker said he is not keen on the idea of acting alongside other Doctors. Matt and J-Lou (Coleman) will definitely be there.
Martha, I don't know. :( The team seem to be quite secretive...hmm...
"Dear God, I hope not" was exactly my immediate thought... I really found her incredibly dull, whiny, and overall a thorn in my side of a character. She wasn't snarky enough to be Donna and not compassionate enough to be Rose. She was a try-hard of a companion, in my opinion.
I can understand where the character was meant to go.. but the acting just wasn't there.
