A comment in another thread made me think of this.

A lot of games have different "play style" settings for AI, like aggressiveness or expansionism.  There are probably better examples in 4x strategy, but what comes to mind is the different rulers in Civilization.  The strategies employed by Genghis Khan and Abraham Lincoln are supposed to be different, and they react differently to diplomacy.  It strikes me that sovereign abilities would be a great way to "program" your opponent or ally AI.  In addition to the bonuses or minuses, having an ability could also influence that sovereign's play style in various ways.  On the character creation side, it would give the player some control over how his sovereign will play when used as AI in another player's game.

This would also tie in well to vassal behavior (a whole other suggestion entirely).  If you appoint a champion to govern one of your cities, you have to consider his nature and the possible consequences of enabling him to act independently.  Appointing a bloodthirsty warmonger to govern a city could easily cause a war, if you don't watch him carefully.