In an old browser based war game i used to play in their PvP system there was something called vengeance.
Basically how the PvP of the game works is like this. You can attack someone up 70 times every 12 hours. There is actually a timer on the person's profile page, showing the last person to have attacked him and how long ago it happened. Once that timer reaches 12 hours the player is then open to be attacked another 70 times. *This has nothing to do with the idea, just briefly explaining the PvP of the other game*
However, if someone attacks you even only once, win or lose, you get something called vengeance on them. What this means is that you can attack them unlimited amount of times. You can go on non-stop and even hit them 1000 times if you wanted to. This vengeance lasts 24 hours and then the normal rules apply. Unless of course the guy hits you again, vengeance is refreshed.
Currently, when people get into what they would call a war in this game, basically all it is, is a few attacks every couple hours and raiding of some buildings. They just go at each other doing basically the same amount of damage every 2 hours. Both sides just take huge losses and neither one really wins. However if there was something like vengeance, it would make the PvP a lot more interesting and would actually make someone a clear winner of a battle in this game.
For MWs version of vengeance, 5 attacks or raids would grant the opponent vengeance for 24 hours. During this 24 hours if even a single attack more is done, vengeance is refreshed.
EDIT: There could also be some tweaks to the casualty rates when someone has vengeance to make this work out better. With current casualty rates, the aggressor would actually lose more than the defender loses even if he won every fight. For this reason, in most cases it would be unwise to actually use vengeance on someone as you'd damage yourself more than you'd damage them.
My suggested tweak would be to decrease the casualty rate of the victor and increase the casualty rate of the loser. I understand that the current casualty rates may be in place to help protect weaker players from getting beat up on by stronger players. So this tweak would only apply when vengeance is active. Meaning this altered rate would only apply with both sides are actively in a war with each other. This way the person who makes use of their vengeance and attacks, then wins, would actually do much more damage to their target than to themselves. Allowing for there to be an actual winner and for some actual damage to be done.
What do you guys think?