Resurrection stones
The whole situation where you have to kill an enemy sovereign outside of his/her territory has been annoying me for the longest time - it was clearly introduced to help the AI because they cannot manage their sovereign properly.
Instead of using this system, I propose that each sovereign starts with a resurrection stone within his/her possession. Each time a sovereign dies, one resurrection stone will be used up. A few resurrection stones could be sprinkled throughout the world as rewards for high-end quests or dungeons.