Would it be based on real-life traffic congestion or in-game congestion?
We cannot add in time for congestion in-game as people who created routes earlier in the game will have lower taxi times, and if they edit the routes the taxi time will go up.
Then why not a certain airport size will have this amount of added turn around time to aircraft off the start? Say...
350k=15min
300k=12.5min
250k=10min
200k=7.5min
150k=5min
100=2.5min
50k=none
This would help cut down the growth of the airlines in major airports(ex. ATL, ORD, LAX, etc...) compared to less dense(ex. MDW, SAN, AUS, etc...). Helping more balance the world in how fast airlines grow, at least at the beginning.