Assuming that the sovereign's ability to fight will be based on magic, there needs to be an option to transmutate into ghastly creatures as well as summon them. We can currently summon a troll or other beast, but I would ask for a whole spell tree devoted to using magic to make my Sov something beastly.

There is even much space here for the Empire's Magic. Imagine a Sov that decided to take "fallen" to a whole new level. He would maybe get an ability to turn into anything he has defeated in battle or could dopplegang something he is currently fighting. He could also maybe spend essence to get these special magical abilities in the form of spells or super rare quests as it may be. The fact is, to rule an evil empire you have to be the strongest one in that empire. Becoming a vampire or wherewolf or even man-eating plant to do that is only logical.

For the Empires it would also be cool to see some Mortal Kombat style soul stealing as a loot function that could restore their mana in addition to the current way. Maybe they have a slower regeneration as a trait in the character creation screen with the potential benefit of being able to absorb an enemy's soul.