Things mandated by law in the US: horns, engine hoods, windshield wipers, mirrors, airbags, tire pressure monitors, backup cameras, seat belts, bumpers, anti-lock brakes, automatic emergency braking, electronic stability control, LATCH systems, mufflers/emission control systems, headlights, tail lights, stop lights, and turn signals.

Additionally air intakes may not be more than 4 inches in height, steering wheels must be circular and at least 13 inches in diameter, and tires must not discharge materials towards the rear of the vehicle.

That covers most of it.