Till know you only can mod graphic elements and names of airlines / shops / etc. At the moment cou can not mod anything gameplay related.
Could you open up gameplay elements to modding? The absolute dream would be, if you could store all gameplay relevant variables in json files.
Contracts: You should be able to modify the contracts costs, e.g. fuel price. I guess that you have some kind of base value and add a variable for the price.
Fees: you should be able to mod the fees limits and when fees are estimated green / amber / red. Also how much green / amber / red will influence an airline decision to use that service.
Stores: size requirements, payments
Passengers: probability to use a service like toilet, shop, restaurant.
Airplanes: Passenger capacity
Staff: payment/h base value, probability for skill levels
vehicles / procurement: Prices, speed, …
I don’t think that putting all the hardcoded variables, like the examples mentioned above, in a json config file would be much work and it could help people like me to tune their airport ceo expericence.