How this will all come about is unclear, but it appears as if it will simply be just a fun episode imagining what it would be like if this fan-favourite version of Holmes and Watson existed in the 19th Century.
“The special is its own thing,” Moffat confirmed, making it clear that this won’t drastically change the format of Sherlock moving forwards. It also won’t tie into the events of the actual series because, “we wanted to do this, but it had to be a special, it had to be separate entity on its own. It’s kind of in its own little bubble.”
Shooting on the Sherlock Christmas special is nearly finished, and we should find out when it will air later this year. A fourth season is also being planned, but with Cumberbatch and Freeman both juggling busy schedules on the big screen, when that will actually get going remains to be seen for the time being.