So, basically, there's very little incentive or reason to fly smaller regional jets and combi aircraft once you're able to afford larger aircraft, unless you are actively engaged in roleplay or "working" for an airline. This results in very few reasons for visiting some of the smaller airports in this game, as larger aircraft generally are not able to accept a job to these airports. This makes sense, but also means that players who play the game for the purpose of advancing economically (so most players) would only really visit the smaller airports within the starter area, and then ignore most of the smaller airports in the outer regions due to a lack of available jobs for larger aircraft.
This could be addressed by assigning values to estimate an airport's expected traffic and remoteness. For example, Enoksen is a fairly remote airport, but the size of the airport and infrastructure present suggests that there is at least a reasonable amount of traffic that flows through that airport. Bonuses can be added to jobs that serve low traffic airports in remote locations to incentivize more service to these poorly served airports. Combi aircraft should receive an additional boost on routes with very low expected traffic, as consolidating the passenger flight and the cargo flight would be highly desired in a route that struggles to fill even a single flight. The effect of this boost should decrease to zero as expected traffic increases, however, since combi aircraft make no sense on a route busy enough to support dedicated passenger and cargo aircraft.
In short, subsidize jobs operated to extremely remote airports that are often ignored due to their lower capacity limits, and subsidize jobs operated by aircraft types often not sought after by most players, but still serve an important role irl. Most people beeline for the largest transport they could find before getting their SST license, and just operate the same routes between the same large airports over and over.






