A quick rule proposal:
Capturing a hexagon for your element is an action that prevents you from doing anything else that turn.
You may only capture a hexagon for your element if there are no players from any different element on that hexagon, otherwise the action fails.
Reasoning: Makes it more difficult to capture hexes for your element, and requires players to work together to force the defending team out whilst capturing it.
Hmm.. yeah that could work. But the reason why I chose that there would be no capturing, is that I think the whole point of capturing is kind of illogical. I mean if I go to an area with 1000 tanks, and there are no other people there, I would say that the area is mine, without the need to capture anything.
Then again it could be like "setting up camp" or something like that.
As an Element's control extends across more and more hexes, it becomes less efficient to collect resources from them. Any hexes that were not originally owned by the controlling element only produce the minimum amount (usually 1) of electrum, regardless of what it originally produced.
Reasoning: This reduces the positive feedback loop that appears when one team gets on a roll and starts capturing lots of land, making them more powerful and their enemies weaker (which allows them to capture more land, and so forth).
We already have something similar. Elements get double revenue from hexes that "belong" to their element, half revenue from hexes that belong to opposing element, and 1:1 revenue from all others.
We could also have a "corruption" system used in Civilization. Basically it means that you lose revenue when you have lots of land far away from your capital, but I don't know how to implement that in WoE without going too mathematical.