Moriarty is dead, and here comes another villain. The upcoming season is expected to explore darker themes, compared to the past seasons. It was speculated that Mary will eventually die in the series, as it was her fate in the short stories.
In "Bad Boys" Joe Pantoliano's Captain Howard says "Just do what you do; only faster." That's the exact pep talk that you imagine Gatiss and Moffatt having with Cumberbatch and Freeman as they return to Baker Street for the 4th series of "Sherlock."