Something about Wolves of God reminded me of older notions of rulership. I don't know if it was the emphasis on the non-hereditary nature of earldom, where an earl's son might not also become an earl if he hasn't accomplished anything. It might have been the distinction between folkland and bookland, where folkland is the king's to grant temporarily but still governed by the traditions of the people (and returned to the king and folk on the grantee's death), with bookland, land granted in perpetuity, very much the exception. It might have been the discussion of having multiple small kings in some of the realms.
But it has me thinking about rulership of peoples, rather than of places. "Arthur, King of the Britons!" Not King of England, but King of the Britons, a people, a culture, first, and Sovereign Over All England second. King in the North, not King of or over the North. This isn't purely a Dark Ages anachronism either; as recently as the 1850s, Otto von Bismarck tried to get the Kaiser to adopt the title "Emperor of the Germans" or "The German Emperor" rather than "Emperor of Germany".
(Electoral monarchy is another one of those fun twists that is forgotten in the popular-culture conception of kingship, but which ties nicely to kings of a folk, elected by the leaders of subsets of their people).
What would a domain game aimed at folk-kings rather than place-kings look like? Heroic deeds cause you to accumulate loyal followers from among the people who you've helped, who provide you with a small amount of income or can serve as warriors. You lead them into the wilderness to the promised land, or to take the lands of their ancestral enemies, or against a crumbling empire (German barbarian PCs vs hobgoblin Rome?), and once your people have a homeland the income that they provide you increases. Sort of a cross between ACKS' congregation rules and Wolf Packs and Winter Snow's rules for accumulating a tribe, but scaled up and then bleeding into the domain game.