I think Slappy was the closest thing to a recurrent villain to the whole brand. He was Goosebumps' take on Chucky pretty much, except with the swearing and Brad Dourif's amazing voice work. Honestly I only read one or two of those books, I guess I just never happen to come cross too many of them.
I used to be a massive Goosebumps fan as a kid.
The thing is, the books were never connected unless they explicitly said so in the title--and even then, R.L. Stine played fast and loose with continuity. Computer games and such muddled this point a bit but even they mostly just made vague allusions rather than outright confirming "all this shit takes place in the same world."
I've heard the recent Goosebumps movie (and equally recent video game that coincides with the movie) fucks this point over though, and also that the Goosebumps brand got revived and
now the novels are all explicitly connected. Fuck that shit though--if it wasn't a thing in my childhood, I don't have to acknowledge it!
..... getting back to FNAF, honestly a part of me prefers to ignore all the spinoff shit, only accept whatever is actually in the games, and just kinda draw my own conclusions from what I see. Done that way, FNAF actually makes a fair about of sense. Most of the lore questions answered by theories or by the novels are really, things that don't matter one whit.
Like going back to Pizzaria Simulator--even if you have never heard of Afton, Henry, Remnant, or Journey From Darkness: Strider Returns, if you get that one specific ending its still easy enough to figure out that the whole thing was a trap intended to put an end to the franchise and the killer animatronics.
.... Plus there's my own oft-mentioned suspicion that the storyline is (and has been even since the later Scott period) a case of make-it-up-as-you-go, where anything that isn't explicit in the games is something they can just change on a whim. Even Matpat has ventured the suspicion that Security Breach's story was overhauled just to prove his theories wrong.... and Matpat's probably correct about that. There is really no point to theorizing anymore, under these kinds of conditions.
Then again I've never been a huge fan of "Theorist Baiting" anyway. For how many horror games do it, does that actually translate into sales? I gotta imagine its about as effective as "Pewdie-bait" was--which I recall hearing, most of the time getting pewdiepie to play one's game didn't necessarily translate it into getting lots of sales.
(Incidentally probably says a lot that Pewdiepie was one of the first youtubers to admit FNAF was becoming boring, all the way during the FNAF4 days).
.........
Someone mentioned Slenderman and his loss of popularity. Funny thing, Matpat recently mentioned that in a theory too ("The END of the Backrooms?" over on Film Theory) but he attributes it to something else: Slender Man got a movie with a major studio behind it. A guy on the internet might not care if people make games or such on his intellectual property, but a big studio probably will. Matpat predicts the same will happen to the Backrooms, that people will stop using them because a big Hollywood studio has the rights now and nobody wants to go up against those lawyers.
Kinda makes me wonder about FNAF and if maybe this was part of the impetus for the Fanverse Initiative.