Allow more than 75 flights:
AirPort CEO currently limits flights to 75 when a large ATC got built. It should be possible to further increase the flight limit.
One approach could be to allow administrators deal with regulations and thus increase limits, or to have other research leading to an increase. Another approach could be to increase by 75 for every tower built.
Why it should be implemented:
Having researched night flights and built an airport with 20+ stands, there are no planes most of the time.