It would be cool if we could have reappearing opponents. Ex If a race is conquerd by a race of an opposing alignment, the conquerd planets would slowly move to have an organized uprising against thier opressors. The event could be stopped or mitigated by building improvents such as the secret police center. Also with the indroduction of ground forces into the game, we could choose to exterminate, enslave or meerly occupy the planet (think total war). Extermination would eliminate any threat of uprising and yield a bunch of cash, but the planet would be decemated and economically hard off past that point. Enslavement would add a production boost to your empire for x number of turns but makes your planets vulnerable to uprisings. Occupation only makes conquerd planets vulnerable to uprisings (midigated if conqueror and conquerd are same alignment) but otherwise the planet is economically productive. If enough planets were to be teetering that were originally part of the same race, that race would reappear as a rebel faction. (one with possibly modified behaviour). You would also be restricted by alignment. Good civs could not exterminate or enslave planets, only occupy them, but would be far less likely to have planets rebel.