The plan is that eventually, polity territories will be based around regions instead of mindless expansion as you say. But the idea is that primitive societies did not initially have a concept of territory. They would just expand and occupy as much land as they needed. And a tribal territory is nothing but an abstraction of how far a particular tribal culture has expanded through pure migration and assimilation. Once more complex intercultural interactions like warfare and trading come in, then it will make more sense for polities to decide and declare that a piece of land, and all of its resources, belongs to them.
Empires, and other complex forms of governments, will depend on the implementation of the delegation mechanic. Once a territory grows too big, it will impossible for a single polity to directly govern all of it. So it will delegate parts of its territory to local provincial governments. Governments that rule over multiple provinces will be know as superpolities. And there will be two main kinds: Federations and Empires, where the first will appear when multiple polities band together to form a single federal government, and the second will appear when a single polity exerts it's dominance and influence over multiple other polities.