I think small planes would be super nice, and especially if there are some game mechanics to go along.
In career mode you could start with almost no money and have to take only small private planes in your small grass airfield. I am imagining a pilot keeping his plane there in some hangar or parking spot. Then he can come to the airfield and fly it for a while. Small planes like this require fuel but not much else.
You will get money from selling the fuel and refueling services, potentially some maintenance and also for the rental of the hangar or parking spot.
As you gain more money you can build a terminal and check-in desks and a baggage handling system and start taking on commercial flights.
And now that I think about this, the initial "airfield" version would be a perfect pre-alpha for the game. Much more limited scope without all the complicated queuing systems, baggage handling, check-in and so on. And at the same time it would allow testing the UI, contracts, general airplane routing, people movements, refueling operations, building system, etc.
What do you think @Fredrik?