Civ 5 actually didn't do too badly with this once you get Ideologies. A civ like the Netherlands or Belgium with only a couple of medium sized cities would actually survive quite well if everyone in the region followed the same ideology. If everyone in the region has identical ideologies, mutual declaration of friendships & friendly with neighbours etc, that is sufficient But generally civs like Belgium are best represented as CityStates - they do offer a great possibility for a smaller civ to thrive based on citystate alliances. After the 'Holy Roman Empire' functioned in this way - it wasn't a centralised entity but rather a loose coalition of allied citystates. I hope that does remain a strategy in civ 6.