Revise how much excess demand is available after the "shown" demand is filled. Limit the number of hubs a player can create (20 hubs worldwide I can wrap my head around. 20 hubs in EUROPE is a bit much and excessive) If a player who has established a route is challenged, leave the burden of gaining new passengers to the challenger. If I have route reputation in the green, I shouldn't have to nuke my revenue to keep my yields at 100%. A stock system is a good idea. For example, before you can purchase/lease widebody aircraft, the "investors" must approve it. To keep share prices above a certain price point, you must reach specific passenger experience goals (green IFS, green legroom, be more on-time than not). Actually enforce the congestion fees/penalties that are supposedly part of the game.
The airline with route reputation in green should keep the most of passengers no matter how much a spamline lowers its prices, or at least, a good reputation percentage should make it way harder for spamlines to take all your passengers. That should encourage realistic configs, low utilization times and good service rather than cramping up the plane and trying desperately to use up a plane in 5 different routes so as to leave no idle time for it. The great problem of this game is that when you have a good airline with good reputation. Then there comes someone with that "me-wants-to-have-teh-biggerest-airsomething-in-teh-world" attitude, offering scam IFS, maxing up the utilization times and offering double or even triple the demand, they reduce the price a few bucks and all your reputation means nothing while all the passengers flock to the other airline. They may even reduce business class prices to coach level and coach even lower making it impossible to fill at least 90 percent of your seats without equally having negative profits