I think demand is already high enough, seriously. I don't comprehend people complaining about demand in this game. You see airports like ATL with an annual movement much higher than in reality.
Last week, in Tempelhof, ATL offered 1,261,016 seats to USA airports, representing a market share of 87.04%. This means
1,261,016/0.8704 = 1,448,777 seats offered in ATL (only departures)
A year has 53 weeks.
So, this means:
1,444,777*53 = 76,785,211 seats offered per year from ATL
Multiplying by 2*0.8 = 1.6:
2 — seats offered from ATL*2 = departures + arrivals offered
0.8 — hypothetical LF% of 80% (Tempelhof global LF was at 83% last week)
122,856,338 passenger per year! (current AGEX level: 772; normal)
Isn’t it enough? Doing the same calculus for other airports (using LF of 80%).
GRU = 59,169,562 pax/year (~35 mi pax/year in reality)
MAD = 97,159,484 pax/year
JFK = 97,859,068 pax/year
HND = 97,910,967 pax/year
PEK = 116,630,511 pax/year
Tempelhof global passengers per year = 97,707,605*53 = 5,178,503,065 (real data for 2012 = 2.9 billion)
This means this game has a global demand 75% above reality. This game already has a natural high demand.