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.