Stephen et al... Long time player (ID 8829) dipping my toes back in again. Since 2006 I've played several of your competitors' games, got an MBA and learned a little bit about programming. So I've come up with some ideas that I think would be easy to implement, help challenge the game, slow down the speed of growth etc...
Terminal costsI assume that in the game, a Terminal and a Concourse are basically treated the same and lumped in under the term "terminal". Atlanta for example has one terminal but multiple concourses, while JFK has multiple Terminals that each have their own concourse.
Assuming it's a "one term fits all", then the costs to build and expand a terminal are grossly unrealistic and contribute to the runaway growth. It only costs about $46,000,000 in the game to build a terminal at the Heathrow or Atlanta which is a a fraction of the costs of the REAL terminals both airports built or proposed in recent years.
Heathrow's Terminal 2 cost €2.3B and took 4 years to build.
Atlanta's new F concourse cost €1.2B and similarly took a few years to build.
I'd recommend making €1.5B the benchmark price for the largest airports and scaling it back from there. Similar to larger planes taking longer to build, larger terminals should take longer to build as well. The 747 takes 168 hours to build (7 real days or 7 game months), so a terminal at Heathrow should take ~288 hours (12 real days or 1 game year).
Aircraft dataCurrently there are several different says that aircraft data is displayed. In the Brokering menu, you can see the multiple engine performance data and their prices, however in the Purchase menu, you do not. Similarly, the aircraft search page does not include the engine variants, which can be dramatic. I've always been a cost conscious/ROI player and that's how I've usually crushed other players way back in the day, so for me, the ability to see fuel usages and pilot/flight attendant requirements would help me make better decisions. Currently, I've had to build my own spreadsheet to track all of this which probably gives me an unfair advantage.
Expenses & WagesSame thought process as above, expenses need to be itemized or better able to identify. As I mentioned above, it's hard to easily find pilot and flight crew numbers but I assume they are lumped together under "Staff Expenses". But we're running two pilots 24 hours a day in perpetuity which is unrealistic. Perhaps for every 8 pilots, you automatically have to hire another 2 man "reserve" team. Same with flight attendants. This will help drive up costs and slow down growth.
Maybe go one step further and have a union contracts for pilots, crew and mechanics that need to be negotiated every 2 game years?
MaintenanceEven the best engines need an overhaul around 5000 hours. Again, we're flying these things non-stop in perpetuity. You could make it tedious like some of your competitors, or just have it so that a plane goes out of commission for one (real) day every 5000 flight hours... Or if you want to make us manually come in so we're engaged in the game, have it stop flying at 5000 hours until we come in to tell it to go to the shop.
I've got other ideas but if you've read this far, you deserve a break.