What I'm having a hard time figuring out is why they would bother with the whole "I have returned to have my vengeance" angle with a character we've never seen or heard of before. Not to say it isn't possible to introduce a brand new character, fill in his back story with a plausible motive for vengeance and make the whole thing work, it just seems... odd.
It's very easy to do, in a way that makes the film self-contained for people watching it as a one-off.
The pre-credits sequence is a spectacular mini-episode in which Cumberbatch's character apparently gets killed. The main movie is set a few years later, when he returns to take his revenge.
Not saying that's what they're going to do, but it's almost as basic an approach to plotting as Act1/2/3/4.
As I said, it's certainly possible, perhaps even not all that difficult. It still strikes me as odd if that's really all there is to it. If my other theory is correct, that Cumbebatch is not playing the ultimate villain in this movie, then all bets are off.