I think if we somehow can incorporate 'war fatigue' to the game, we can limit the war but if we want to add this similar to the real world, the game will have to change quite a lot. I am sure the devs don't want that since it will require a lot of coding.
I think the easiest solution will be enforcement like Europa Universalis or CK2. EU uses 'war score' to enforce your demand. Maybe we can do something similar. I think we know from the accumulated game history people hate losing and mostly would rather see their realm destroyed than admit their defeat. We can maybe allow people to set how the long 'peace' period would last. Of course we do not want to prevent people from declaring war during that time but there should be some kind of punishment for dishonorable actions such as breaking peace before it expires. Maybe 25% honor and prestige loss for every member of the realm wouldn't be so bad.
Or if possible maybe the dev team can program something that would track the casualties, maybe when the casualty rate reaches certain percentage compare to the total population, it would pop a message saying something like 'war is draining too much manpower' which makes the realm lose 1~3% drop in production realm wide every day? Reducing the number of recruit you get won't be bad either.
Building off of this, make all war decs identify specific regions as targets, and limit their number to something like, say, five (arbitrary, could be anything reasonable). When all those regions are no longer under the control of the target realm, the war
immediately ends. Then stick some cooldown period on the function so that one realm can't repeatedly war dec another.
Almost by definition, any mechanic we add to limit wars is going to be punitive, so we may as well define the limits clearly.
