The fact that Moriarty appears only in The Final Problem is interesting in the fact that he has become such an important part of the Canon in this single appearance. He has joined Watson as the most identifiable, and arguably memorable central characters from the original stories. A layperson, unaware of the Canon's many recurring characters, may not know the name Lestrade or Mrs. Hudson, but James Moriarty has taken on a life of his own. Due I think in no small part to his presence in William Gillette's successful play, Moriarty has turned up in a number of pastiches. Various writers have contributed greatly to the mythos surrounding the Napoleon of Crime, giving him a background and life of his own. And it's this decision which invariably leads to the problem with the Professor.
It makes perfect sense when adapting the Sherlock Holmes Canon to the screen to introduce Moriarty earlier than The Final Problem. If he truly is the organizer of half that is evil and nearly all undetected in London,chances are Holmes and Moriarty would interfere with each other. The Professor even acknowledges as much when he meets Holmes for the first time saying:
"You crossed my path on the fourth of January...On the twenty-third you incommoded me; by the middle of February I was seriously inconvenienced by you; at the end of March I was absolutely hampered in my plans; and now at the close of April, I find myself placed in such a position through your continual persecution that I am in positive danger of losing my liberty."These tantalizing tidbits allude to earlier matches of wits between Holmes and Moriarty (only one is ever chronicled; The Valley of Fear which merely finds the Professor's name dropped). To the screenwriter, it is simply too much not to chronicle earlier adventures, or slip Moriarty's name into an earlier adventure. This technique makes sense, except when it does in Series 1 of Elementary when Moriarty is mentioned once and then promptly forgotten until the finale. An equally excellent realization of this foreshadowing can be found in Granada's The Adventures of Sherlock Holmes which shows that Eric Porter's Moriarty was the one responsible for the Red-Headed League.
And that,I think, should have been the end. We saw Moriarty kill himself in The Reichenbach Fall, there is no way that he could have survived such an injury. If Series 4 decides to go with the so-called "second-brother-named Moriarty" theory, then I hope that Gatiss and Moffat handle it well. The idea of having two brothers both named James Moriarty is a humorous explanation to one of Conan Doyle's original lapses of logic, but I daresay an actual realization on screen would be a bit silly.
Now, you can easily refute my argument saying that Moriarty is just such a fine character,so it would seem illogical not to use him. He's also the most memorable villain from the Canon, and the easiest to make into a multi-faceted being. While such characters as Charles Augustus Milverton and Baron Gruner are just as memorable and interesting, they do not have the same weight to span an entire series arc. Also, in defense of Sherlock, when it was first produced, there was no idea that it would become so successful and spawn a number of follow-up seasons. Gatiss and Moffat were throwing everything they had onto the page in effort to take full advantage of the opportunity they had.
Nevertheless, here is my challenge to any future Sherlockian screenwriters out there (and how I wish I had the opportunity you have). When you go to plan your next Sherlockian series, perhaps keep Moriarty off-screen for a while. Don't try to integrate the Napoleon of Crime in from the word "go." We fans will know that he'll probably be along at some point in time and this'll only add suspense. What's more, if you decide to keep Moriarty around, you won't write yourself into a corner by either having him imprisoned (see Elementary) or maybe dead (see Sherlock). If you wish to ignore my challenge, that's fine, and I won't be offended. But perhaps now the time has come for a change. Remember, you don't have to play every card you're dealt.