New prominent decoration (perhaps hanging skulls on a post) that is unlocked after (x) number of successful revenges. Lets players know that this player will likely try to get back at you.
An alternative to this (to future-proof it so everyone doesn't have it down the road) is to decorate the town hall with skulls if the player has 5 successful revenges in the past month (or whatever makes sense based on stats).



