Goosebumps/WMG

In 50 years, the Goosebumps series will be studied in school literature courses.
Whole doctoral theses will be written on how these stories are masterpieces of the horror genre ranking with Edgar Allan Poe's work and The Martian Chronicles.
 * We can hope, can't we?

I Live In Your Basement is the REAL end to the Series.
The REAL Twist is that the Entire Series is All Just a Dream by Keith. Anything written after that is just more dream scenes.

All Goosebumps titles exist in a multiverse like The Dark Tower.
The mascot Curly can travel between worlds a la Rod Sterling. He was the voice saying "Goosebumps, viewer beware you're in for a scare" in the opening theme of the TV show.

The Series is meant to be a Parody of the Horror Genre
It makes sense. Doesn't it?
 * Except of course when the stories actually are scary, right? Fridge Horror is quite common. The earlier stories are more of the type with an emphasis on direct horror. And there are the CYOA "Give Yourself Goosebumps" that in the same book may be a parody with one ending, a subversion with another, a double-subverted discussed parody that then subverts its parody nature by becoming a Dangerously Genre Savvy ending...and even just a couple endings that are played perfectly straight. Not to mention the use of ghostwriters, meaning canonical dissonance.

The girl on the front of The Curse of Camp Cold Lake is meant to be Sarah, not Della
She fits the description of Sarah a lot better along with being perfectly opaque, unlike see-through Della. And the surface looks like clammy cold white skin more than bone. Perhaps Sarah was killed by the snake, her body dumped in the lake to hide the evidence, but whatever magic was in the lake kept her from decomposing too much and somehow her soul stayed with her body/corpse and we got what we see on the cover today. As a result, she'll either be High Octane Nightmare Fuel or Fetish Fuel or perhaps just Ugly Cute in her own way.

The world where "Attack of the Mutant" takes place is Earth-Prime.
The creator of the Masked Mutant is psychically tuned to the Mutant's universe, what he thought was his imagination was actually events that really happened. The Masked Mutant created an interdimensional portal which sent him to Skipper's world.