Many people have said that Sherlock just wasn't the same after the hiatus. Or Sherlock wasn't the same after the hiatus. Or both.
In the 1920s, it was how the post-Reichenbach Sherlock Holmes might have been fictional, as several bits of him don't match up to pre-Reichenbach Holmes (Father Ronald Knox in the essay "Studies in Sherlock Holmes"). In the 2010s, it was how seasons three and four of BBC Sherlock just didn't live up to the promises of seasons one and two to so many fans.
In that 1920s Knox essay, he referred to a "Deutero-Watson," a second biographer of Sherlock Holmes who wrote some of the tales and not the others. But to my mind, even as I first read "Deutero-Watson," I was evolving Knox's words on Sherlock not being the same to a post-Reichenbach "Deutero-Sherlock." Sherlock Holmes did die at Reichenbach Falls, and then was replaced by a second Sherlock Holmes.
Many a Sherlockian has toyed with this idea, over the years. A Vernet cousin. A twin. An actor. A clone. Such theories are of limited use when considering the original Holmes Canon as we tend to love all sixty tales at this point, even the runts of the litter, and don't want to suppose that even the Sherlock of "The Mazarin Stone" was not our Sherlock.
But as I reflected upon such theories and applied that filter to the issues of BBC Sherlock, the idea of a Deutero-Sherlock seemed much more sound.
We are never told exactly how Sherlock Holmes survived the Reichenbach Fall. We know that someone who looked just like him came to the graveyard while John mourned at his grave. We know that Mycroft pulled a shaggy spy from the field to take up Sherlock's place in London. And we know that John Watson seemed to want to punch Sherlock Holmes a lot more after his BBC hiatus, starting immediately. And Mycroft did use that phrase "the other one," which we were supposed to accept meant Eurus . . . but did Eurus ever seem like anything close to another Sherlock?
As messed-up and secretive as the Holmes family eventually was shown to be in "The Final Problem," the idea that there was an additional sibling, a twin for Sherlock is not that far fetched. Or, failing that, an MI-6 agent that Mycroft Holmes made into a replica of his brother. (That would certainly explain all the spy stuff coming in so hot and heavy.)
But, as tempting as a second Sherlock might be, there is that price that must be paid . . . a price earlier Sherlockians have always been unwilling to accept to explain a few character changes. Does one give up two seasons and a Christmas special to hold that the pure Sherlock of seasons one and two was not the man who came back post-hiatus? Both were played by Benedict Cumberbatch, 'tis true, but it tends to leave John Watson in an even messier place than he is at the end of season four, even though he may not consciously be aware of it.
Has anyone wrote a fic of post-Reichenbach Johnlock where poor John discovers Sherlock is truly not the man he remembered? Surely they must have a this point. It would seem material enough for a whole sub-genre.
As much as my mind goes back to this little theory of a Deutero-Sherlock, it never stays long. In fact, mere moments after I finish this off, I'll probably be back to a single true Sherlock in my headcanon. But it's always an interesting question to raise.