Here's some new ideas (I love making these up!):
Make a Game where dying effects the player more than having to restart. (e.g: You get different endings based on how many times you died)
Make a Game with multiple Endings.
Make a Monster that isn't evil. (e.g: Kissy Missy in Poppy Playtime 3)
Make the Player Question their allies. (E.g: Players aren't sure if their friend is really helping them, or if they're just manipulating them.)
Add an Easter Egg to your favorite Horror related Game/Book/Other.
Base your horror game on things that exist in real life! How can you make your reality scary?