(1) Most people fitting larger tires are choosing wider offset wheels to clear the weird suspension knuckle (which limits tire size). That puts the tires sticking out past the fenders (illegal in many states). A set of stock-looking fender flares would no doubt sell really well.
(2) Software allowing users to turn off the aggravatingly loud HONK HONK HONK the car does when you open and close the door with the motor running would be a godsend.
(3) Full Aluminum skid plates
(4) heavy duty roof rail attachments allowing a roof capacity of more than the factory's meager 150 LBS limit would allow people to use roof top tents and carry heavier roof loads.
(5) aftermarket locking differentials (this is probably a pipe dream, no one made anything for the WK2)
(6) White and dark tinted front fender reflectors (there are tint/sticker options, but no full replacement that retain reflectiveness, and white/clear would look better on lighter cars)
(7) piano black protection film kits for the center console
(8) rear hatch area nets for the sides and for the actual liftgate, or an emergency medical kit to attach to the liftgate (so it's always accessible).
(9) second row matt that covers the carpeted area around the cupholders (no other options offer it)