Just last week, a teaser was released for the upcoming Christmas special of Sherlock. From the looks of it, it will take place at the time when Sherlock once lived back in Victorian times. This would be a great special, taking Holmes and Watson.
Hell, I get a kick out of watching Miss. Hudson and Watson arguing about the stories that he's written. That he had to grow a mustache so people can recognize him. Yeah John, the only way people will recognize you is by your mustache.
Here's the teaser.
The Special will be it's own thing and won't be part of the fourth series. I don't know if this will air on Christmas on PBS. It would be awkward if it did with Sherlock and Doctor Who airing at the same time. But it would be great to have them air in one night back to back with each other too.