I've already been playing out all possible scenarios in my head, or at least all those I can think of.
I am torn, part of me is really intrigued and excited about finally meeting CAM. I expect some amazing acting and a really suspenseful storyline, part of me is just scared as hell. I really loved the last two episodes, the candy floss, the emotional warmth!
I think Sherlock is the one who miscalculates something, maybe he was so smitten by Mary (whom I love so far) that he overlooked a few important indicators he most likely would have picked up if he hadn't been trying so hard to fit in with John's new life (like the word "liar" popping up in his deduction). I guess so...
Or he possibly had a hunch all along and kept it to himself waiting for how things will play out, with Sherlock one is never certain. I also thought he was a bit overwhelmed by Moriarty in the fall, and I was wrong. But then the infallible Sherlock Holmes screwing up big time when it comes to something really important because he is a bit blinded by his emotions (the very thing Mycroft warned him about) would be a great storyline, and I think this is what we will get. There is an undertone of that already to be felt in "the sign of three".
But I also don't think that Mary turns out to be a true villain, just think there is a rather unpleasant secret in her past, maybe she was supposed to spy on John for some reason and then ended up falling for him, just one possible idea...
I can't imagine the fourth series starting with another reunion between John and Sherlock. We just had that. Repeating themselves is not what Mofftiss tend to do.
My guess is Mary dies and somehow possibly redeems herself before dying - through sacrifice?
I really like Mary, but I can't see her be part of a fourth series. The show is about Sherlock Holmes and John Watson and their friendship and their cases in the end, it will never turn into the John, Sherlock and Mary show. I also think John somehow has to make his way back into 221B in the end.
Mycroft, well, I think people of Mycroft's position know about people like CAM and in a way also need to work with those people to a certain extent. One should not forget Mycroft isn't a selfemployed secret service, he more or less directly represents the British government and will always have to act in its interests. So I guess he is not pleased having his little brother fight the very man he is possibly trying to keep happy and in check...