Perhaps the "sphere" could vary by population density.
So if you're out in the middle of South Dakota your sphere, at its limits, would be well over 100 miles in radius. But if you're at NYC, that sphere only radiates, say, 10 miles. And within that sphere, as you radiated further from the center, the population would be less likely to travel to your airport until it got to 0. In a perfect world you'd consider geographical barriers, but that would be difficult. Considering political barriers would also be nice, and would be easier than geographic ones, but still quite difficult I'd imagine.
The purpose would be to represent greater transportation options as wellas the fact that, the greater the density, the more resistance to travelling further (traffic, time to travel X distance, etc).
The problem then lies in what "density" do you use. You can't use that of the area immediately around the airport, because an airport in a rural area near an urban one could end up with a large sphere that fully-encompasses an urban area (or in many cases, several urban areas if they were close enough and the airport rural enough to be granted a large sphere).
As an aside, we have "Airport Size" in the game now. While we should start with the figures that are already in there now so that "large" airports are already large even when the game starts, that figure ultimately should be variable based off the number of routes/flights flown out of an airport (or, even better, the number of pax going through). And certain costs need to be a function of that number -- particularly landing/pax fees. Perhaps it could even change how often or how many slots are added each year. This would make smaller airports attractive for start-ups, particularly later in the game due to their lower costs as well as having available slots, but it would prevent an airline from jumping into a municipal airport smack downtown somewhere that happens to have decent infrastructure, and turn it into a major bustling airport without having to pay the higher fees of those in 'larger' airports. If an airline really was filling up every slot, the airport would have to increase their fees to cover the operating and capital improvement costs necessary to provide service.