I just have a question. While it may be a ton of work to get right, is it not possible to calculate how many planes an airport can handle (like you said 300 for LHR as an example) based on the maps of the airport. They should show all the remote parking spots, and obviously the number of gates. There is also an element of guesstimate involved, since parking airplanes can be done in different ways to fit more in. Here in Denver, for example, I've seen aircraft stacked on gates occasionally (small commuters on a widebody gate), and on the remote spots, some parked backwards and others pulled in forwards. Just some ideas regarding those tarmac rules you are considering, which I think would be a great idea. I remember back in another game me and another airline were based in Vegas, and I always wondered where those 400 planes would park.
I haven't opened a base up yet, but I do think I miss the 4 leg routes. However, I don't think you should reintroduce them until there is some method for calculating connecting passengers. 4 legs could destroy someone's legitimate hub, and unless that hub get a LF bonus for connecting, it would hurt the concept of base airports.