We need to make it lucrative to fight against the big guy on the map and not the underdogs, at any current time of the day, that means that if one faction owns only 3 keeps, killing that factions players, and taking their last keeps/resources shouldn't give that much if any ap at all
The two factions with most map control, will still have the same values of ap gain from being killed or taken over a keep.
That way this will spread out the pvp more, and the 2 vs 1, wont be against the underdogs anymore, but rather against the people that have more map control.
In another mmo, they have something like this, but the underdogs will get a powerful helper to turn the tide for a while instead