Figured I'd make a serious post to discuss this. I don't think your sector-based plane limit will work, because people will simply fly into that sector from the next one. It's not a real limit. You can limit only the number of planes that up from a field -- and how do you do that? By period of time? It cannot be an absolute number, because often you'll find 2 bases close to a sector, and the planes from both (and GVs, don't forget them) will be in that area.
I heard an idea, and for the life of me I cannot remember who posted it (I want to say Karnak, but I don't know). I think this idea better serves the whole ENY balance issue.
The idea was to increase the factories, towns, and "strat" stuff in an area, and then tie the fields to the geography. You have to flatten (bomb) the towns, cities, and the entire area to capture it, and once you capture a zone, you gain the fields in that area. Naturally this would mean smaller zones and more importance on carpet bombing.
As a side effect, this would change gameplay. It would change the emphasis. While you would still get some furballs, any one horde would be countered with another horde, because the second you don't protect the area (the "front" if you will) you lose several bases at once. Leading to a more dynamic front line, this means that countries have to defend vigorously their strat. I think that means any buildup by enemy is seen as an immediate threat and countered with its own build-up.
I wanted to bring up a civil discussion (make note of that, all others who are reading this) about possible gameplay changes in the future, beside plane limitation based on ENY. This particular idea change the focus of gameplay to the point where hordes are automatically countered with other hordes, so the "horde" itself doesn't need to be limited on a sector-by-sector basis.
What do you think?