I think the "demand" engine needs to be changed a bit regarding SOME secondary airports.
Some examples I'm talking about...
DCA vs IAD......Realistically DCA should have MUCH MUCH Greater demand than IAD. In the real world, DCA is the PREFERRED airport, it is DRASTICALLY closer to Washington DC than IAD. HOWEVER, IAD has more flights because it has A LOT more slots and much longer runeways. DCA has high demand, but low slots. I think demand from DCA needs to be drastically increased in the game. [In real life, ticket prices to DCA are much higher because demand is high, supply is low due to slots]
MDW vs ORD.....Once again, MDW is actually CLOSER to Chicago than ORD. However, runways at MDW are smaller and has a lot less gate space. Considering the airports are only like 10miles apart, the demand for domestic cities should really be nearly exactly the same.
DAL vs DFW.....Once again, these airports are only 10miles apart.
Another suggestion would idea would be to make demand based on a "region". So demand from Chicago to NYC is 5000pax/day. All flights from Chicago(MDW,ORD) to New York(EWR,JFK,LGA) should count towards that demand.
This would promote not having EVERYONE fighting for the top 10 airport slots when ORD and MDW have same DOMESTIC demand for example.