May I suggest something. It's not something new and was in LotR as well..
Well, I propose "Rules-Constitution-Edicts" system something like this:
  • Rules - They will always be OOC and they are .. well.. Rules!
  • Constitution - IC laws that are discussed and accepted before the game starts. These are laws that must be followed all times. Only high ranking player can propose Constitution Amendment. Here can be mentioned what settlements England(example) considers theirs and thus those settlements become fair game for players to take from AI or another player.
  • Edicts - Laws that are in effect only between two Council Sessions. Penalties for braking them could be mentioned in the Constitution. Edicts can not contradict Constitution.


This way you can slow down expansion and make all those settlements be worthy and most of all.. there will be lot of fighting for taking/keeping those settlements.

Hope that what I just said makes sense!