Note that I'm suggesting ideas not considering your current plans, so sorry if they don't help or are implausible:
1. Bureaucracy/Organized Administration/infrastructure- This discovery is not necessary for polities to form, but if a Polity wants to expand its size. I.E administration cost may cover how much population a polity can manage, while this discovery manages how far in size the borders can be maintained, therefore a High Population, Large state cannot hope to maintain unity unless they have discovered Bureaucracy. Bureaucracy cannot be discovered unless the polity has knowledge of Agriculture, and at least 5% of the population farms, the level of Bureaucracy within a state is determined by the Bureaucracy level of the Primary cell of the leading faction, or if possible the capital. Bureaucracy levels can exist in Non farming polities which may happen to succeed from its parent faction, however the Bureaucracy will rapidly decline, causing the territory to collapse. If the level of Bureaucracy is critically low, the rate of clan creations and Independence will skyrocket, increases rapidly over time until the Bureaucracy is large enough to maintain the state borders.
2. Colonial Administration- Colonial Administration is necessary for Polities to directly colonize or incorporate land beyond lets say 3 tiles or a proportionate amount of Kilometers. In order to discover Colonial Administration, Polities must be capable of Agriculture, have a Bureaucracy, have a large amount of social organization, population above 50,000 and a certain, but high level of ship technology. As the level of Colonial Administration increases, so does the chance and range of colonies increase. Note that colonial administration is not required for human populations to migrate, but rather for polities to make large overseas conquests. Additionally, Colonies are managed by clans which will be generated to control the new land, making independence within a colony realistic, so a clan halfway across the world cannot declared independence and somehow own half the land in the colony. When a colony achieves independence, if there are significant amounts of natives (foreign populations that have a high rate of prominence within the newly independent colony) the colony will get an event which allows them to change its language/culture to that significant native population, buttt, if there are significant amounts of populations from the original polity (colonizer) then they may leave the newly independent colony.
Please keep up the good work, I will try to think of new ideas, god speed Dr.