Reading the comments, here would be my initial and broad solution. Details to be discussed:Airport capacity
Capacity of an airport is limited by many things, and in AWS the model for this limit is mainly the runway capacity (= slots, runway slots). Other limiting factors in real life are ramp space and terminal capacity for example.
So I would propose combining the suggested terminals feature to the current slots feature. Having terminals alone as the capacity metric is not realistic, and airlines rarely build or fund own terminals outside US, but we can merge these two to create a suitable solution here that has both real-world elements and solves the airport issues in the game. (the current slot allocation is not realistic either)Elements
Two elements are needed: runway capacity and terminal capacity.
Runway capacity will be calculated based on the current system, but added with the airport expansions where airport authority can elect to build new runways and expand the airport's runway capacity when it becomes limiting. There's a separate thread of this too with basic details here: http://www.airwaysim.com/forum/index.php/topic,21347.msg268408.html#msg268408
(the idea's details may have to be changed a bit for this but basic idea remains: airport can expand its runways every X years and thus grow larger => more capacity). So this element is dynamic, but slow - like in reality as new runway planning and building can take time (how much time, we should discuss - taking into account 70-80 year game spans)
Terminal capacity will have many elements and possibilities and it is also dynamic, but faster than runway dynamics. Basic idea starts with each airport having a single terminal operated by the airport authority - this is the same like today. So for any small airport with very little traffic nothing changes. But we will introduce a possibility where players can open new terminals for their exclusive use, and possibly also rent this terminal space to other players (or alliances; details to be defined).
Terminal will have a defined max capacity per hour like runway slots. Terminals can be expanded by player input (money), and the airport authority would also expand their own terminal if needed.Usage
Base idea would be that local airlines have to make their own terminals once they pass certain size, and all visiting airlines use the "default" (airport authority) terminal, unless they wish to make a contract of using player's terminal (similar to fuel contracts etc; just have to make easier interface tied to route opening screen). Each terminal can have defined costs for handling, pax transit etc. Each terminal would also have a quality rating based on some values (to be defined) and this in turn would affect the pax'es desire to use this airline (terminal).
What each airline does inside his own terminal would be then his own matter; longhaul, shorthaul, whatever. However we could allocate the 'gates' (parking spaces) at the terminal also according to the 4 plane size classes so that you'd have different available capacities per plane type. For example total capacity of 10/hour could mean 2x very large or 10x small (just examples).
Staff numbers would also change; you wouldn't need handling staff for example anymore if you buy terminal services from airport authority or other airline, but staff would grow when your terminal grows etc.
Capacity usage % of slots and terminals would also affect to flight delays. Terminals could operate up to ~120% of their design capacity but delays would grow much when >95% capacity is exceeded. (this is advanced part, not necessary)Costs
Slot costs would be radically reduced as your main costs would go into operating the terminal. There would be no slot limits per airline (apart from the hourly limits etc) but the limiting factor should be the size on your terminal. I would also presume the overall runway slot numbers would need to be increased.
Or alternatively the relative size of the terminal vs. other terminals would be also your max. share of the total rwy slot allocation at that airport. But these are details.Bases
We could also eliminate the base 100 plane restrictions, since it's limited by terminal capacity now (and past certain size you'd need the terminal, or you can build it right away too). And this feature would also make this "number of aircraft based an airport" feature work: http://www.airwaysim.com/forum/index.php/topic,25319.0.html
..which is basically the same thing what is talked here.
For non-based airlines flying here the airport authority would make sure there is always runway and terminal capacity to them (for runway capacity this has to be thought well too).Update
This all in all is a massive update and could not be implemented to v.1.3 most likely which is bad since we got 13 months of games left there etc. Eta for building/testing 3-6 months+.
All combined, this would still mean that airports cannot grow exponentially and wildly (runway capacity) but much more dynamics would be introduced with much chance for players to interact and make changes to the environment themselves too. On bad side this could possibly be complicated and also if the runway capacity does not work it would still mean issues (however at some point the rwy capacity will limit anyway). Also how to deal the game start situation is another thing to think about too.
This is what I though of today, and is a very rough idea. Please discuss but do not focus into very tiny details so far. (I may have forgotten something here too)