Read this essay?
http://shadowfireflame.livejournal.com/64605.html
Do it.
I agree with shadowfireflame; this series seems to highlight Mycroft more than before.
He plays a major role, just like Mary.
Mary definitely causes problems; she's the stone that brings down everything. There are too many signs in the previous episodes to come to a different conclusion. She's being blackmailed, and she'll - whether deliberate or unconsciously - become this series' doom. She'll lead to the final problem in this episode.
Sherlock will be in trouble; series three focuses on him more than any other series before. While we saw events through John's eyes and occasionally got a glimpse what Sherlock was up to, we never truly saw the events from Sherlock's perspective. The audience remained just that whenever he was "on stage" without John. We were spectators.
In series three, Sherlock becomes our main focus; we experience The Empty Hearse and The Sign of Three through his eyes. Many have had problems to accept the change of style; they needed to rewatch the episodes to get comfortable with it. And that's why; we get more from Sherlock than ever before. The final problem will be a situation in which he will have to choose. And more than likely the decision will be harmful to himself.
And I believe it'll be Mycroft that covers for him. No matter what he stated in the trailer. I can't see Mycroft dying. I can see him lose his job, though. Or risk it.
Here I'll take a right dive into it: I can see Mycroft try to kill Magnussen.
There's a darkness in him, different from Sherlock. Mycroft is more deliberate, more detached. The only thing he has ever been emotional about is Sherlock; remember A Scandal in Belgravia? He was angry; at the same time, though, he was unable to outright blame his brother. He felt responsible for him, for the "mistake" Sherlock made. He was about to cover for Sherlock then, too.
Milverton, the character Magnussen is based on, was killed by someone he blackmailed. We haven't seen enough of him for his actual demise yet, but I think the cliffhanger this time will include a solution to the Mary dilemma, and it will show Mycroft take some sort of action, despite his words towards Sherlock. I could imagine him pointing a gun at Magnussen. I could. It would be the ultimate character exploration; clearly, he wouldn't shoot. Haven't seen enough of Magnussen. But there would be enough questions hanging in the air. What will make him retreat? What will that mean for his career? Magnussen is a powerful man. Will Mycroft be able to land a stalemate? Or will it cost his job?
I'd go so far to state that Magnussen is after Mycroft from the very beginning. Sherlock is Mycroft's weakest link, his weakness. Why would Magnussen be interested in Sherlock otherwise? Unlike Moriarty, he is not interested in riddles. The Bonfire Incident merely is a test run; to see which baits Sherlock will react best to. What makes him dance.
