I had an idea of some things to do with family gold.
Allow the family itself to have control of recruitment centers. Have these centers be able to send troops out to family members for a massive premium, but the center is tied to the family itself and cannot be used by anyone else, does not have an upkeep, and cannot be disbanded as militia. Think of it as a private bodyguard that your family pays for.
Allow people with family gold to use that gold to purchase more votes in elections. Would give more opportunities for people to suspect elections of being rigged and cause more strife within realms.
Last thing, embargoes. People with incredibly wealthy families would in theory have massive trade routes everywhere in the world. Allow families to embargo realms at a penalty to their family gold, would impact tax gold of the realm itself by a percentage. I know this one would require a lot more coding and a lot more effort to make workable but i think it's pretty cool.
