Great to see progress!
Would it be possible for polities to want to form their borders according to region borders in order to make polity expansion somewhat more rational? So as a basic precursor to more advanced behavior, polities will reach a stage in which they care more about regional conquest rather than 'mindless' expansion, in which they will pull together resources to conquer or colonize specific regions. It'd be interesting to see how regions could be utilized to step into more complex and intelligent routes to expansion on part of the polities. Perhaps regional expansion could be the first thing to filter tribes from 'empires', in that the first distinction is a change in the goals of conquest, in which tribes sporadically expand, but empires want to dominate regions, and that the empire will decide were to expand based on the average attributes of a region. So for example an empire will look at all adjacent regions, then decides to NOT invade a region with a higher rate of arability, due to the fact that a more powerful empire controls the region, and instead the empire decides to invade a more isolated region inhabited by weaker tribes.
Anyways good job!