The BBC have released a trailer for the one-off Victorian Sherlock Special which premières on New Years Day.  The story looks like a classic supernatural murder mystery Sherlock has become known for but is actually not based on a Conan Doyle story, and is created and written entirely by Stephen Moffat and Mark Gatiss.  They'll be another short wait before the arrival of season 4 near the end of 2016 which will see Sherlock back in modern day London.