You have to build semi-realistic coasters - anything ridiculous almost immediately will result topped out fear. Loops, screws, etc - anything that turns people upside down adds a lot of nausea. Fear is caused when the coaster is going to fast for the section - for example if you're going 100 miles into a turn, fear is going to skyrocket.
You can literally have a coaster that throws people into 14G turns it just has to be at the right speed or you have to balance that fear out with other sections of the ride.
I'm disappointed that the water rides pull so much attraction - the ride I really put a lot of work into perfecting people don't really seem to have THAT much of an interest in.