These may have been proposed already, but I have two ideas, revolving around Events and Oil.
By events, I mean events that have a certain probability of happening based on some decisions. For example, a strike if your employees aren't happy and ground your planes; plane maintenance issues if you don't maintain your planes correctly (would require more maintenance controls), plane crash in very rare cases, causing customer demand to your airline to go way down and you have to build it up.
Gas prices change over time. Gas prices were different in 1958 than 2008. I think these should change. I also think you should be able to stockpile oil to lock in low prices.
These would help expand the game beyond simply constant expansion.