For example: North-East Dwilight. The success of the Morek Empire has killed the fun of the region. The only war/event that broke the eternal stagnation was Summerdale's suicidal crusade. And it had a very predictable end.
I am undecided which opinion to have on this matter. You bring up a very good example, but in my (quite biased) opinion, Morek's success is the result of many repeated mistakes of its neighbours. Summerdale could have played it VERY differently, if they suicide that's not Morek's fault, and I wonder therefore whether game mechanics should somehow limit Morek's success when it comes from players choices.
On the other hand I agree with the fact that there is a concrete lack of game mechanics tools to help rulers give to any wars a more interesting outcome. Again I believe this can be solved by players (see for instance Fontan's fate on EC, they have been given a chance, too bad they wasted it), but some additional game mechanics would definitely be helpful in having interesting peace treaties.
I however also agree with most of Indirik's remarks, it's not very easy to let players play as vassals and like it.
