The Grand Moffat just tweeted the news:
Sherlock returns, Sunday,Jan 1st, 8.10pm, BBC1. First the woman, then the Hound and then, of course, the fall…
The reference to “the woman” is the classic Holmes story involving Irene Adler, A Scandal in Belgravia. Sherlockology has a spoiler-free review of Scandal that will whet your appetite!
The fall, I assume, is a reference to Reichenbach.