My idea for a LEGO Ideas set is a Haunted House Roller Coaster (which you could find in a LEGO theme park). At the ticket booth, the minifigures buy a ticket with which they can enter the roller coaster train via the entrance platform. After the first corner the train dives directly into the haunted house to reach the highest point of the roller coaster. After the first descent the train rides across the balcony of the haunted house past my homage to one of my first LEGO sets: "Black Monarch's Ghost". Then the train goes back into the haunted house and drives past all kinds of creepy elements and figures. After the last descent, the train drives out of the haunted house to arrive at the platform where the next batch of minifigures is already waiting for their ride.
The model consists of 2710 bricks (which immediately makes it my largest MOC). I think this set would be a great addition to the existing attraction/theme park themed LEGO sets.
NB. Due to technical issues in Bricklink Studio, I was unable to put the train on the rails. Hence, it stands for the model.