...you could already start one discussion: considering the civs available in civ5, you all can already think of a group of appropriate city-states for the earth map. Say, 15 or 25 cities (depending on map size, which is still unknown), divided into "present in 100% of the games" for necessary ones (Madrid? Carthage?) and "present in X% of the games" for less important ones. You could suggest a list of these cities, taking inspiration from RFC independents, writing spawn date too.