It might be a good idea to have more different types of flight in the game besides GA and CA. Mostly, these will still fall under GA rules, but might require the player to provide other services.
Charter flights could for example be requested before you have built a full-fledged CA terminal. This could work with giving the security road building an extra job. Operate it with 1 or 2 security staff and from the outside world a bus or limousine would enter the airport tarmac by road and deliver the passengers to small aircraft on let us say asphalt or concrete stands. A charter flight would be counted as a GA flight for runway purposes, would have small planes (to implement) and would require the services ramp agents, fuel (either ACGas100LL for Cessna-types or JetFuelA1 for small private-jets. Security will be done at check-point (we might expand or upgrade this so that the bus can park while being inspected). Charter flights should provide a higher parking fee somewhere between 500$ and 750$ thus gaining more than a GA flight but less than a CA flight. You might reserve stands for charter flights exclusively.
This could be done e.g. like this:
a) Charter Flights require security access via vehicle checkpoint, fuel services and ramp agent services together with an upgraded stand. (Requirements: asphalt or concrete; GA accepted, Service Car access)
b) They are counted as GA so if you switch off the GA feature, you won’t get them. Ramp agents will access the stands via service vehicle as if it were a remote stand. Depending on the plane, the required fuel will be either AVGas100LL or JetFuel A1.
c) Charter flights will appear as one-shot flights in the flight planner, but under GA flights. UI might have to be modified to switch view between CA flights and GA flights. I could see a second page of flight planner working here that only cares for your GA stands. GA flights automatically register there without COO help and you see when space is blocked. Assign a GA stand to fix time and services for your charter flight.
d) Charter flights can be of any small aircraft type.
e) Payment for a charter flight could be a fixed parking fee of 500$-750$
f) PAX will be delivered directly via outside world vehicle (hope that works on small stands), which can be buses or limousines and enter via vehicle checkpoint.
g) Maybe baggage is handled directly by the ramp agents which unload directly from bus to plane. Baggage handling fees apply at reduced rate of 5$. No passenger handling fee.
h) Change the airport vehicle checkpoint so that it can house up to 2 security agents and works like a security desk for vehicles. Let it have a parking lot for 1 vehicle which is checked by the officers. The more passengers the vehicle has, the longer this will take. Security skills apply to reduce time. This feature is important to enable direct boarding without the need of going through a terminal.
Could we label bigger than small stands as GA to accept medium charter flights?
Could we later implement military flights as charter flights (mostly eye-candy with camo vehicles and planes)
Could extra plane models for charter flights like the Learjet be implemented to simulate private jets?
Could we add and lease hangars later to have business partners operating charter flights from our airport?
Why should we have this feature?
Charter flights are something that comes evolutionary between GA airport and regional CA airport and gives players an incentive to keep GA and granularly grow their airport with smaller steps to have cash intake for building bigger structures. Might be more interesting for players starting on hard difficulty. You would only need to upgrade stands to better quality and have a staff room for security and ramp agents before you will start the full infrastructure of a commercial airport.