There has long been a trope in science fiction that every moment creates new universes. If you decide to turn right, a "turn right" universe springs up. Turn left and an entirely different "turn left" world arises. This seems like a whole lot of energy being expended just to give importance to minor choices, but it's a fun little extrapolation game to play. And, it really does happen.
Yes, it really does. Maybe not in our really real world, but in fan fiction world?
Season one of Sherlock happens. A thousand fan fiction universes spin off. Season Two of Sherlock happens. A thousand more fan fiction universes incorporating the choices made by the showrunners for that season spring up. And so on.
Can you imagine what alternate fan fiction reality generations would have come had Victorians had the internet?
A Study in Scarlet sees print. Fanfic writers spin off stories about Watson's disabilities, romantic entanglements with characters we never heard of, Sherlock advising foreign detectives . . . .
The Sign of the Four appears. Watson dumps previous romantic partners for Mary. Holmes boxing and addiction fics run wild (and boxing-related addiction fics). Sherlock's multiple siblings start appearing, and he might even be the eldest brother. Sherlock and John work with Toby the dog on a regular basis.
The Adventures and The Memoirs come out. Sherlock Holmes is suddenly dead. Dead, dead, dead. Afterlife fan fiction happens (as it actually did, with John Kendrick Bangs.) Do the fans produce stories where he didn't really die? Probably not, as they have dozens of episodes of new material to play with. Irene Adler, Moriarty, and Mycroft all come into play. Except Mycroft is just a government auditor who is too lazy to be a detective. Mycroft fan fiction takes a big hit, as who wants to write about an auditor?
The Hound of the Baskervilles doesn't affect much.
The Return of Sherlock Holmes brings Sherlock back to life and futzes up almost a decade of fanfic and headcanon. Mycroft is still an auditor. Milverton shows up and we start getting darker, criminal-act Sherlock . . . or just feeding of those who already had written him dark.
The Valley of Fear is a problem in any world. Watson knew about Moriarty?
His Last Bow. Finally, Mycroft gets to be the British Government! Sherlock the spy fanfic begins, if it hadn't already. Watson's preface also alters any retirement fanfic, heavy on the rheumatism.
The Casebook ends the Canon. No real alternate timelines spin off this one, because it always just seemed like fanfic by the author anyway.
Each publication, like each season of Sherlock, would have brought changes for those who bravely charged ahead into the lives of Sherlock Holmes and Dr. Watson. And they would have had plenty of time to do it in those days, with hiatuses lasting such random lengths as three, one, eight, two, ten, and four years. My meagre extrapolations above don't begin to touch on the possibilities that could have come.
But that, really, is the cool part. Sherlock Holmes and Dr. Watson have always had so many possibilities in them. Universes upon universes, and many, many more are still out there, waiting.
Turn right. Turn left. Head off at a fifteen degree angle. It's all good.