I just came across a MASSIVE and I mean MASSIVE ShamWay Foods Manufacturing plant for the first time in my last RWG. This is easily as big footprint wise, if not bigger, than a small rural airport. The town I grew up in had a small airport that serviced single engine planes. I can tell you the entire thing was smaller than the Shamway foods building minus the runway.
We have skyscrapers that have probably close to a hundred rooms in them and thousands of blocks. A small airport wouldn't even be as big block count wise. The ground level foot print seems to be the main problem.
If someone wanted to get creative, they could certainly create this and export the POI, the MAIN issue I see with these being hard to implement is finding enough flat land in RWG to place the runway. The rest of the building is not that hard to create given the enormity of the Shamway foods plant and skyscrapers already in game.
Any large airport simply wouldn't make sense unless you are in a city that would support such a structure with its economy.