I'm working on an HRE in my mod that has a special relationship (with very positive influence modifiers via lua) with a City State (the Free Imperial Cities). When HRE captures a CS or major civ capital, that is transferred to the Free Imperial Cities. For every city in the the Free Imperial Cities, the HRE gets bonus points for Great Person, Golden Ages and Gold generation. I'm thinking about modifying this for use in a more general way rather than just for my scenario so that the first CS conquered by the HRE would become the Free Imperial Cities (I'm not certain if a city state can be renamed in such a way, though). Perhaps this could even be expanded such that units/science or other yields could be provided based up the number of cities owned by the Free Imperial Cities. Perhaps, like Venice, settlers might be disallowed... haven't gone that far in play testing it.