Game design is about experiences.
What kind of experiences do you want your player to have. laugh, cry, engrossed, mentally stimulated.
How do you want to convey the feeling with freedom, restriction, pressure.
How do you want to present the game. Fantasy, future, abstract weird.
idea's are easy. You can even take real life jobs. Here's an example from a prior job I used to do. Security
What does security do. They prevent theft or harm to clients.
How do they do security. They walk around the work environment checking doors and monitor for danger.
What experience do I want to do. I want to mentally stimulate the player
How am I going to convey it with restrictions.
How am I going to present the game..... futureistic.
So here is the thought.
A level represents a Floor or Layout of a building structure. There are hallways and rooms.
You get X amount of floating patrol droids in a level.
You can mark a path for the droids. Droids will look into each door delaying them from gliding through the hall.
At exact periods of time a crime will occur at the same spot and time each play through.
The player must plan a plot through the hallways that will let the droid/s encounter each crime.
The more crimes prevents the higher the ranking the player recieves.