Horror is one of the most creative cinematic genres because it allows plenty of room for cheesy jump scares and gore, as well as intellectual concepts and drama. You could pretty much add blood to anything to make it creepy, but these Redditors take it one step further by imagining what would need to happen for non-horror to be remade as horror.
There are already plenty of terrifying non-horror films out there, but they pale in comparison to the eerie concept of turning Matilda or Wayne's World into a horror phenomenon.
From Redditor /u/acute_winnipegosis:
People inexplicably go missing in Truman's neighborhood. It's been happening for as long as he can remember. Only Truman seems to notice or care. Is the town being stalked by a serial killer? Are the police in on it, or is Truman losing his mind?
Or, could the truth be even darker?
From Redditor /u/Hoods_77:
I can see it taking a Carrie route, where she [takes out] her abusive family. And some of the stuff that the Principal was doing was really dark. She could end up [slaying] the principal and liberating the kids, only to go all power hungry or something.
From Redditor /u//ButImNotNice:
An R-rated Scooby movie where they think it's just going to be an old man in a mask, but it ends up being an actual serial killer (or zombie swamp thing, I'm not picky about it).
[Editor's Note: According to the film's own screenwriter, the film was originally given an R-rating, and had a darker tone overall.]