I don’t mean to have helicopters or airplanes. I actually don’t like those. And outside world is not multiplayer. What I meant is like the harbor in RailRoad Tycoon 2. Just a place where goods appear and you ship them inside the country. Same could happen for the airports. The company won’t have planes or anything, but the airport will be a source of goods. They just appear there, as if they are arriving from another continent, and the player will deliver those goods by train to other areas.
Harbors and Airports can be a source of raw materials (yes, harbors can receive oil, etc) and also demand materials. Just like in RRT2. Like a place where the industries can export.
Also its a very good way to make available things that aren’t available in the map. For example, you want to make a desert map and in this case you won’t have any farms or even wood. Those can come from the harbor/airport.
Airports can also be a source of people and mail, as well as demand those items.