Rather than have to capture almost all of a country's fields to reset a big map, how about if you only had to take their three sector fields? This would shorten the time a country spends tied in a corner under attack from both other teams. Even when their last sector field is threatened chances are there would be plenty of other fields to fly from.
Also it would alter the way the current maps are fought, with thrusts towards the sector fields, rather than the war of attrition we get now.
