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.
As far as transitioning from current airports, it may not be clear if the airport is runway capacity limited or terminal limited. In order not to be stuck where we are now, it may be a good idea to assume that airports have greater runway capacity than the current system indicate and they are terminal / gate limited.
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).
Let's just hope that airport expansion of runways will add so much capacity that most airports will be limited by terminals / gates (that while costly to enlarge, they will be under player control.
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).
I think the transition to new terminal will be challenging. What might work is that the player will have to start with the smallest terminal, with incremental upgrades with ability switch flights from common to private terminal. This way, the space at the common terminal will free up gradually. And, if the players happened to grab 100% of, say 0500-0700 slots, that his new, small terminal cannot accommodate, the player would not have to stop flying.
Anyway, the idea is for the player to have access to 2 "sources" of slots all the time, and enough incentive financial (cost) and CI/RI/TI (Terminal Image, for lack of a better word) incentive to use his own terminal. So perhaps, using common terminal would give the flight TI of zero, using his own terminal would give him TI based on the size / quality of his terminal.
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.
Hopefully, that expansion would be slow, such as runway expansion. Since every time player moves out, there is more capacity left at the main 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).
Might be better left for future versions.
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).
I think this - dealing with sizes of gates, as far as plane sizes they can accept might be even a bigger change than the building terminals, since, in theory, it should apply to the common terminal as well. It might be best left out of initial version, and just stick to slot is a slot, gate is a gate. This might be something that would have to be left for future, after the main mechanics are worked out, and apply only to new game worlds.
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.
But the economics / improved image should favor own terminal - at great deal of capital expense to build it.
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)
I think the delay system could use a re-write, so that it actually simulates aircraft flying, challenges it is facing, and cumulative delays / ability to catch up. Best to handle that separately.
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.
Yes, probably a good idea.
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.
Sounds like a good idea.
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+.
I would foresee is something minimalistic, simple to get off the ground, with testing being done by creating a copy of existing, running game world. Testers would start (continue their existing game) without a terminal, using one common terminal, with an option for the player to build his own terminal. Otherwise, the test would just take too long.
As far as implementation, I would foresee something minimalistic: One slot screen per terminal as opposed to one slot screen per airport. Player with a terminal would be able to switch between the 2 screens to view the slots at each terminal.
Route screen would have an option enabled for a player with a terminal (disable for player without a terminal), allowing him switch the flight back and forth (with edit, setting a check box, save). Plus, overall runway capacity limit would be enforced. That's pretty much it, apart from working out the staffing, costs, and some boost to potential LF by the Terminal Image. So everything works exactly the same for the player without the terminal, only changes when the player decides to build a terminal.
One potential change might be that when a player builds a terminal, the capacity of the common terminal might need to shrink, by some 50% of the size of the new terminal, and same for each expansion of the player terminals. We would not want to have 80 evening slots at LAX common terminal, with each airline there being able to add maybe 40-60 of his private slots and we end up with 300 evening slots at LAX, all flying red-eyes... This way, there would be some check on the overall capacity of the airport. So if 2 players add 45 each, for 90 total between them, the common terminal would shrink by 45 from 80 to 35 slots.
When the test game world is started (copied), players all without the terminal would first check if all the mechanics work pretty much the same as their existing live game world. Once that is verified, the option to build terminals would be enabled, and the players would have at it. The advantage of using a copy of existing game world is that it will have a number of large, established airlines, without players having to start from scratch, go for weeks before they are large enough to be able to test the feature.
Maybe more than 1 cycle of this test might be needed, starting with a fresh copy of a game world and re-verifying no changes to non-terminal play. Then when testing is done, all game running game worlds would get the new code base (since it was verified and re-verified it does not make any changes to player without a terminal) and then the terminal option would be enabled in all game worlds.