For me everything would be about how the audience feels. If I can drag them into feeling like they're going through the movie with the main characters, then I could scare anyone. It's easy to scare teens or easily impressed people. All you need is a guy in a mask slicing teens up somewhere. That's not really scary. It might be cool to watch sometimes, but it's nowhere near horrifying. What's truly scary is when you get drawn into the movie, and you forget that you're watching a piece of fiction. You gotta build an atmosphere that scares people throughout the entire movie instead of the last 35 minutes.
To do that I'd have a main cast that is different from the norm. No generic leading blonds. No implant girls having sex with Abercrombie *****e bags. No token characters. My main cast would include some ordinary looking people, and they'd actually have a reason to be in the same situation. Chances are I'd set the movie in a place where people work. It's easy to have 8 people in that situation together, even if they're from different backgrounds.
My antagonist would have to be something that can attack at any time, but doesn't for other reasons other than to make the movie longer. Ghosts wouldn't really wait around to start the killing, they'd probably just start killing when they feel like it. If someone has a beef to get revenge, a la your standard slasher fare, then they'd really just whip out a gun and off the people instead of toying with them. So my antagonist would have to be something or someone that will brutally murder my cast, but has enough reason not to do it so quickly. I think witches(Suspiria) or zombies(Romero) would be a good antagonist to build up the fear. Or I could go with something completely original like The Tall Man or Freddy Krueger. The bottom line would be to have things make sense, but to build suspense at the same time.