While not confirmed, the image seems to be a teaser for the upcoming Sherlock Christmas Special that will air later this year. After all, Cumberbatch’s Sherlock Holmes and Freeman’s Watson are wear Victorian-era clothing. This connects with Sherlock co-creator Steven Moffat’s earlier confirmation that the Christmas Special will turn the clocks back to Sherlock Holmes’ original Victorian era, when author Arthur Conan Doyle first wrote the character (The BBC’s Sherlock, for those who don’t know, takes place in modern times). And that moustache Freeman is sporting certainly doesn’t match any style from this century.
While plot details on the special could elude Holmes himself, Moffat has told Entertainment Weekly that it will be a standalone project with no plot connections to Sherlock’s three prior seasons.
So they're making fanfiction of their fanfiction, essentially.
I don't know why I'm being so snarky, when we all know I'm screaming internally. This fandom never dies, it's like a horror movie villain. We're just resting, I swear we'll be back for every sequel, season, and Christmas special.