The scariest haunted house I’ve been to was held at a state hospital for the mentally ill, near where I went to college. How did we know a patient didn’t escape in all the confusion? You probably don’t have access to a mental hospital though. :)
I like the suggestions above, and will only add one thing. At one haunted house we visited years ago, there was a long line waiting to get in. As we stood in line, a couple of people in creepy and disturbing costumes were walking around. They’d come up to you and just stand right next to you, staring at you. At first you’d giggle, then start to get creeped out. They didn’t jump out and try to scare anyone or touch anyone. Just stand there. Silently. Staring.
You could build a maze, have it be very dark with fog and scary sounds. Each time someone makes a wrong turn in the maze, have someone waiting to jump out and scare them, or just stand there moaning or talking gibberish and acting kind of insane. If possible, make some parts of the maze moveable, so you can separate people from their groups. Suddenly finding yourself alone in the dark would be very frightening!