The game isn’t really designed for code based modding. Currently we use the UMod Framework to inject custom code on top of the original code.
And we have to convince the devs to support the mod loader as the UMF isn’t a good long-term solution. See The AirportCEO Mod Loader - What, Why, and How
If we ever get to the point to implement complete new features such as crews, families or cargo handling facilities is hard to say.