A nice source of income (and initial expense) would be to be able to create parking.
The fees could be controlled separately for simple drop-off/pick-up, hourly parking or long term parking.
Some airports in India also have parking space rented out to Taxi/Cab operators to park their cars between drop-offs and pick-ups. A similar solution can be implemented when the cab model is up and running.
The Devs have said that this will be implemented in the future.
They ever mention if we can create one-way roads/bridges to connect the arrivals and departure areas like in the real airports where mostly highways connect it with off-ramps showing arrivals- terminal A this exit, Arrivals Terminal B…this exit, Departures this exit…airport exit…bus and taxi drop-pffs? Would be cool if we can place parking garages and drum up some tourism business by offering rent-a-car service (perhaps one level in the parking garage could be dedicated to rent-a-car and rent-a-car returns.
Airport Parkings have already been discussed on the forums. Please try to use the search function before posting new topics! 
Oops.
I only checked the Ideas section. Did not occur to check the questions section
How to booking airport parking in UK?
Probably not different from ready of the world.
What?
I mean the rest. Why you think it would be different?
This is implemented already, right?
Fortunately, it is!
It is indeed implemented.