Just a couple of random thoughts and ideas:
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1. Demand calculation offers sooooo much creative freedom for all kinds of events and modifiers. Apart from the already mentioned seasonal changes, I mean come on. Throw in some aviation and world history:
- It's 1960. Jet a/c are all the rage. Huge bonus pax demand for 707s/DC-8s, prop demand decreasing.
- It's 1969. First 747s to be rolled out. Huge bonus pax demand for 747 flights for like two years.
- It's May 1979. AA 191 just lost an engine. Worldwide fleet of DC-10s grounded, massive loss in customer trust towards DC-10s.
- 9/11
- etc. etc.
All sorts of challenges one has to deal with in order to keep your airline successful. Maybe throw in some random events.
1.1. Or how about demand modifiers for local produce? E.g. using Tupolevs in Russia, Asia or Africa = no problem, filling up a Tupolev at KSEA = good luck. I.e. country-based demand modifiers for certain a/c makes.
See, everything you throw at us to react to increases the immersion and finally fun of the game. It also gives us something else to do instead of expanding like a late Lord Manderley. Last game in O2, I've left the game for like two months and still found myself in top 3, as there was nothing to do to stay successful.
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2. Make every individual a/c more important:
- I'd actually give a kidney for a more complicated seat config thingy. Let's define individual classes, like Premium Economy. Design seating by seat width/pitch, with pax preferring more comfortable products on long-haul trips. That'll immediately remove those 660 pax 747s. Then add in some bonuses like in-seat USB ports, anything that'll seem sensible.
- Slight reduction of daily available flight time would reduce expansion as well. 20 hrs/day really look a bit extreme.
- DRASTICALLY increase mx costs of outdated a/c. Flying 40 years old Tu-114s should bankrupt you rather quicky. Also see "prop airliners in jet age".
--> therefore keep us updating our fleets, especially for flag carriers. Third-world, charter or cargo airlines should buy off old a/c easily, but today's "I'll buy the Second-Hand Market empty" tactics shouldn't be allowed to work anymore.
- Make petrol savings through new a/c more feelable, thus motivating to modernize fleet.
Same as before: give us stuff to do
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3. Regarding routes, well, I fear to say that dynamic demand calculation may be the road to go (should be a bugger to code, though).
- Airports with many international/long-haul flights should generate demand for connecting flights...
- ... and vice versa, airports with a broad connection of commuter flights should increase the number of potential long-haul pax.
- Maybe some sort of natural growth over long time periods? 2010s air traffic is much higher than in the 1960s, after all. Airbus in particular likes to throw annual growth percentages on their Facebook, so why not steal their numbers.
- Set some limits to how big airports can grow without something like runway extensions happening. IRL, you can't simply build a new terminal and you're free to go. Maybe some player collaboration thingy to collect money for a new runway?
- Pay for runway as well as gate usage. Look, up to the A380s air traffic expansion occured more through bigger capacity jets instead of clustering more and more frequencies. So reward players for intelligent capacity management, don't let us just spam more frequencies as we do today.
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Phew, that's it for now. Sorry if this sounds more like a Santa Claus wishlist, those points just sprang into my mind.