As far as the top 20 large airport restriction, maybe changing it to something that's flexible so the different strategy that can be played with your first airport, and to a greater degree in the country or economic zone isn't limited while still maintaining some level of that top 20 restriction and leave it up to the airline to use their judgment whether or not they will be able to operate reasonably with the remaining slot availability. So it would be....An airline is to have no more than 1 top 20 airport, whether it is your HQ or one of your base. If you started at an airport not in the top20 (country/single aviation market still applies) say KSAN, you may create a base in one of the top20 airports later for example KLAX. If you started in KLAX, you may not create a base at KORD or any of the US airports in the top 20.
I think this gives an incentive to some to spread out, to start at a relatively less crowded airport in the early game.