Not being familiar at all with Cumberbatches work, what if....he is not the reason behind all the destruction we see? What if Wellers character appears in the film and almost immediately kills off Harrison? Harrisons appearance in the trailer is just a red herring and Weller is actually the prime antagonist throughout the film. I don't know if Benedict is a big enough star that this would ever happen or if J.J. has the skills to throw such a curve ball.
Is EBC that big of a commodity that he would not accept that kind of treatment on screen? (like Shatner and cameos)
Cumberbatch is something of a rising star and indeed the show in which he plays the lead (and titular) character has had to wait until he's available before they film the next season. Why would they do that if his character is almost immediately killed off? Hell, he wouldn't really have to put any time in on set for that.
Plus what little is known makes it very clear he will be around for a decent portion of the movie. Would he have been on the movie's poster if he weren't the main villain?