I would like to propose an idea here that I haven't seen before, having to do with robbers.
What if, after a certain point in the game (like after collecting a certain amount of treasures), the kingdom could once and for all completely defeat the robbers such that they do not return all server long. As a reward for completely defeating the robbers, the kingdom would gain a permanent benefit to the kingdom, such as a kingdom-wide fighting strength boost for troops, or else a kingdom-wide resource production boost, or a kingdom-wide troop training speed boost.
The trade off for getting the boosts is that your supply of treasure generation is now gone. The benefit, though, is that now you don't have to waste your time on the pesky robbers and can focus all attention on the enemy. Since kingdoms that opt for the boost are not generating treasures, they have to steal their treasures from the enemy kingdoms.
Where would the trade-offs have to be balanced such that 50% of the kingdoms would opt for the kingdom wide boost while the other 50% would opt to keep the robbers?
I would suggest that it would need to be framed as a decision that kings would have to make sometime in mid-game, and that it is a decision that gets locked in and you can't get out of it without disbanding the kingdom. If the kingdom disbands, the process restarts from scratch and any boosts that were in operation are reset to the original statistics.