To spice up Diplomacy and the course of wars...
What if you could have 1-on-1 duels with enemy sovereigns? The outcome of the duel would result in war continuing or ending, or even one sov surrending to the other.
You're winning the war, but the other sov is higher level? They challenge you to a all-or-nothing duel. You can decline, of course, and keep kicking their ass...but wouldn't you want to humiliate them?
You can send a champion in your place, but then they would be subject to perma-death...