For the sake realism(since I haven't even touched the game yet so I have to base this on my own experience) if a world were to rebell they would most likly plan it out to the point where they controll the primary bases and the major centers as well as a good portion of the military. As with the french revolution, the rebels immedietly took the Basttile witch was thought to be a symobl of the kings supperior might(a big lie by the way) To make matters worse in the french revolution the main milita at the time (ironically the National Gaurd) switched sides and convince Louis to surrender to the rebels. So on an end note in the game ships witch come in contact with rebeling wolds should have a chance of switching sides or at least not like the idea of attacking the rebels(woulden't matter in an emotionless society though) and those ships may switch sides or spread unrest with 'tales' of the crushing of the rebellion.
Also considering how unforgiving humans are to their own kind now just because of nationality (I'm not pointing fingers just stating a point). How would the peoples of an empire who have no common relation to the rebbeling empire who is potitioning for membership react, especcialy if they were at war with the empire they were once attached to?
My thoughts
