An addition to that could be to research rental car parking/garages and add that as extra building options. So start with a (grass) parking lot with X room and be able to expand that, maybe also different parking lots for different rental companies (like on Heathrow, but also depends on the level of (micro) management you want to add). And when time progresses, money grows and airport space diminishes, you can build a garage where all the rental car companies are on separate floors (like on Hartsfield-Jackson). But for that we need multiple terminal floors and maybe even the option to build garages in the ground, so we can built terminals/hangars/stands/etc on top of them.
See that is I think a further level of detail that may not be implemented. It is realistic but might be too much for the game… I would imagine it would simply be some passenger parks the car and flies out. Another random passenger (chosen at random) from a different flight after an x hour gap just walks to the garage and leaves with the car.
As far as my previous comment goes I was saying they will definitely implement rental cars and parking in some capacity. The level of detail and timing is something we do not know
I think it is funny this comes up again as it was posted in jun 18 haha. I think the next voting round we should aim for some smaller features (like this one) so we can have an even better development rate. By that I am not saying they are doing a bad job at the moment