i like HT's idea, it may not solve all the issues, but i don't see any reason to not try it. i think it is a good step to help concentrate forces on a map in a way to promote more actual fighting.
maybe also consider linking fields together. add some strat for those strat guys. for example, maybe at the start you have 5 fields to choose to attack. after you take a base, maybe you get 1, 2, or 3 other options to choose from depending on map, location of field, and altitude. (not added on to the 5 original options). maybe even the amount of players on a side can affect allowable base.
maybe if one team has 30 players, while the other 5, then only 1 base could be attacked on the lower numbered side, while the lower numbered side has more options. you could take it even farther if imbalance is to great than the country with lowest number of players could not have a base taken by a country who has huge number advantage.
just a couple of thoughts, no idea of the time consumption on HTC's part for any of this. but as i said, it's worth a shot, and probably stepping towards a solution. vast