I like some of the suggestions on this thread.
The one I agree the most with is that terminals should be massively more expensive to build. They should also take time to build, say 6 months to a year. Also, I second the motion of limiting the amount of additional terminals that an airport can have. After all, real estate is not unlimited at airports or anywhere for that matter.
If hubs are to be more expensive, perhaps to make things easier on startups, a hub in one's home-base city could be at a discount (say the price of hubs now), as if it were a tax-break given to the airline by the city, while subsequent hubs could cost 3 or 4 times as much.
Eryk's suggestion about leasing companies is great. Even with a C credit rating, an airline can lease many planes. It takes little time to get it moved up to an AA credit rating anyways...
There are also way too many gates at most airports. The airport in my hometown of Kamloops, BC (YKA) has 4 gates, in AE, there are 25. Nearby Kelowna, BC has 6 or 7, but has 50 in the game.
On the flipside, it would be nice to see a larger amount of connecting passengers at hubs without having to have hundreds of flights connecting to it. I don't see why a hub with 7 or 8 fully used gates can't have a large amount of connecting passengers. 7 full gates is 350 slots, which would be 50 daily departures & arrivals, plenty of opportunities for connections in my mind...
I see a problem with having a limit on how many gates could be build, because in the beginning airlines would build 50 gates at an airport (and thats more then I operate even out of my biggest hubs at Miami and New York) which would prevent other airlines from moving into said airport. Also in some airport where all of the gates are leased already you have to build a terminal if you want to operate out of there, or if you want to operate more then 5 gates without opening a hub (some cities that have really high demand you would have more than 5 gates there)