The Big question is, What is actually returning? Digital Spy produced an article a few weeks ago which listed the posibilities, which included, The Master, Ood Sigma, The Face of Boe, Dalek Cann and a few others. But the question is which one?
We will not know for certain until the final two specials, but for now we can guess and cross out the posibilties which do not fit. In my mind it has to be The Master. All the evidence stacks against him but one big thing DT said goes against it. John Simms, who played the Master has been seen playing filming for the special; at the end of series 3 The Master's ring was picked up by a misterious women, which also signifies his return and there is talk of the Timelords returning. However as I said earlier DT said that the 4 knocks is not what you think.
Question number two is, who actually picked up the ring? Here comes a list again of posibilties; The Rani is a human like the Master was and picks it up; Lucy Saxon has planned the scene with the Master, for him to die, if they find them selfs in the worst scenario and she takes his ring and he escapes some how; Sally Sparrow from Blink is searching for the Doctor and comes across the ring; Donna has begun her search for the Doctor after Runaway Bride and finds the ring and well I carn't think of anyone else.
Here is a picture of the possible ring snatchers:
Oh yer and heres is what the Sicik women said to the Doctor in POTD:
Another Master Return Video: