This is a nice bonus feature. If it could be expanded on I would propose that somehow the cities from another civ provide additional unhappiness (and could be more based on civics, i.e. GodKing, Sac the Weak, & Fend for Themselves provides additional unhappiness for cities of other civs, City States, Liberty, & Social Order less). To provide a counter-balance go with an Age of Wonder like ritual (so not rushable) that lets you convert a city to your population based on the distance to your civ.