@Bennie I had a small write up for a similar game idea. I ripped off Cowboy Bebop and the movie version of Jules Vernes' Time Machine with the moon having blown up, raining destruction on human civilization. I didn't go back to 10,000 BC. I shot for more of a bronze age, Conan feel. One tribe was animistic, one was scholarly, one was religious nutjobs, etc. It allowed for layers to come and go. New players are characters that just came in from the wilderness. Players leaving are characters heading out into the wilderness. Just add some communal challenges every month or two to keep things from getting too slice of life-ish - food shortage, wolf pack nearby is attacking, raiders have been spotted, water shortage, etc. Let the players figure out how things will run politically so there is some internal drama as well.
Another idea I would like to see is Space Rome. Feudal lords and ladies in space has been, so lets freshen it up with Rome instead. The setting would be another star system than our current one after humanity had a diaspora throughout space (either generation ships or humanity's space empire collapsed and we lost a bunch of our tech). Space Rome has recently discovered (rediscovered) interstellar travel and is pushing out to other stars, finding other human colonies to interact with. I could see Space Rome having already united all of the star system they are in from the get-go or start with other planets in the star system still independent.
The benefits of this system is the Senate which allows for more players to be involved in decision making, as well as the idea of collegiality that the original Rome had which had multiple people sharing the same position (think the two consuls who had the power to veto the decisions of each other). If one of the consuls suddenly disappears due to RL, you still have another around to get things done. In addition, unlike Space Feudalism where Duke Atreides disappearing results in things hitting a standstill until an heir can be found, Space Rome shrugs its shoulders when Quaestor Atreides disappears and simply elects a replacement.
Let the players drive the storyline and staff just has to mix in the occasional crisis - Space Carthage, led by Space Hannibal, has decided to invade the system; a Space Kraken has entered the system and is attacking ships; Space Pompeii has some seriously bad tectonic activity going on, figure out how to stop it/evacuate the citizens.