Pre S1: Let’s do Moriarty right from the off. We’ll surprise the audience and make him charming. And engaging. And sexy.
Pre S2: Moriarty’s really captured the audience’s attention. We could do so much more with him. But then it would be all about Sherlock and Moriarty, and our show is all about Sherlock and John. Let’s shock everyone and have him kill himself. That’s the end of Moriarty. We can move on.
Pre S3: We’d better have a villain that’s about as far removed from Moriarty as we can. Cold. Vile. Repulsive in his actions. But let’s throw in a bit of Moriarty just to surprise the fans and because we’re not ready to say goodbye to Andrew just yet.
Pre-TAB: It was good to have Andrew back. Damn it, let’s just have the whole episode about Moriarty. But he’s dead, so it has to be in Sherlock’s mind palace. And let’s throw in some sexual tension because damn, everyone can see the UST every time Sherlock and Moriarty interact.
Pre S4: So. Villains. Let’s go for someone that’s even more vile and repulsive than Magnussen. Not that we’re out of ideas or anything. Oh, and there’s Eurus. Who’s basically a female version of Moriarty, but no-one will notice that. Oh, okay, let’s give them some more Moriarty. But make it clear that he’s dead.
Pre S5: So… villains. How about…? Hmm. What about…? To hell with it. We all know no-one will ever come close to Moriarty. He stole the show in TFP. Let’s just bring him back. Alive this time. I know, I know, but everyone knows that we lie. And No body was recovered, remember. Clever of us to throw that line in there. Thank God. Let’s make Sherlock great again.