I don't know if this will be a feature in Humankind, or even if it needs to be (it could create an interesting dynamic, one could build a civ that instead of expanding themselves, it just creates and expands its satrapies) (I'm a Seleucid Empire fan, so I use the term satrapy more XD ) But if it is a feature, I hope its better done than in Total War titles.  In those games, it's never helpful to create a satrap because they still hate you and immediately rebel.  So I think there should be an innate Ruler-Satrap diplomatic bonus that keeps them positive to you while in a neutral diplomatic state (no actions for, no actions against).  But it would be a cool mechanic to play the balancing/diplo game of keeping all your satrapies in line, while still utilizing them to help you the most.

There would also need to be some level of control over one's satrapies.  If they are purely autonomous like they are in Total War, then they usually do nothing helpful at all. But if the player has to micro the satrapy to much, then there is no point in creating a satrap over just conquering the place.  We should be able to give basic orders/commands to the satrapies, but how they complete these orders is largely up to them.

In one of the feature focus videos, the tech tree was shown in the background, and one of the abilities unlocked was vassalization. I hope there are a variety of similarly complicated diplomatic relations, like minor factions that pay tribute or serve as a buffer state within your sphere of influence, or similarly to the idea of satrapies, vassals and puppet states that could be established from cities of other players. If that is allowed, I also hope that the existence of successful rebel states is also possible. But like you said, there has to be some kind of motivator to permitting the continued existence of small states.

Also based on previous focus videos, there is an "Administration" system.

There is probably an admin limit on the number of regions you can control, so that players can't directly control everything they conquer above that limt.