I'm thinking just absolutely crippling ENY at even moderate number disparities. Like to the point where the 110 is the best attacker, and the He-111 takes the place of the lancaster because everything else carries only up to 2000 lbs of ordnance. Want to camp a GV spawn? You'll be doing it with M8's or M3's.
It would also need to be based relative to the lowest number side, not total percentages. Say, the highest side is 30% more numerous than the lowest side, and the third side has 15% more, ENY would be 30 for country 1, and 15 for country 2.
Just tie ENY in a linear manner to the population percentage of your country relative to that of the lowest country, obviously up to a maximum of 40.
And if people log off because they can't horde, and refuse to change countries...... well they represent the worst AH has to offer, and I'm not at all sure losing them would be a bad thing.