[Boomzor]

If left as is, the rich getting richer will happen and probably go hand in hand with double-teaming an empire already on the knees.

This will lead to 4:th empiring and no one can stop that in regards to it's-F2P-I'll-just-make-a-new-account. We don't want this.

A simpler way to implement this:
Some form of tax that scales with the percentage of continent real estate the empire controls. The more you have the less you gain from new territories (or perhaps the resource yielding territories to be exact).
You will still gain resources, but not as much as someone who controls far less then you do.

A more difficult way of implementing (in the line of what Biscuit was thinking):

Have a Train of haulers go from the resource yielding facilities to the continental foot holds. For nostalgia, let's call them ANTs. Either simple AI controlled ones that stick to the roads and chug along while it still have wheels or even let players do the hauling. The "dividence" will not be shared untill the ANT reaches the foot hold and can be hijacked or stolen by the other empires while en-route. Escorting or Hijacking the ANTS could be a nice addition to the mission system, and also points out the logistical difficulties of an over stretched empire.