I really wish I could stop my appointed provincial governors from intermarrying in my Roman Empire playthrough. I mean literally every king-level viceroy was a Komnenos so by the end of the game your typical Komnenoi noble had 14 fingers. Plus I was getting sick of my distant relatives forming rebellious alliances against the main branch's rule. Oh and of course them inheriting each other's land.
When I first started out I had a strict one county per ruler and the governor would usually be the capital county's holder unless I didn't like them or they didn't like me. I would also personally appoint every single strategos in a kingdom. It was a good system. Then they started inheriting more counties, some outside their viceroyalty, which resulted in some really ugly border gore when I switched to the direct vassal map. Also, the recent expansion screwed up my save and now my Exarchs are hereditary Despots. That marry and unite their intended to be separate provinces.
Also, a law forbidding counts from having more than one county at a time would be nice for that extra level of tyrannical, absolute control over every single Count in the Empire. Maybe via mandatory gavelkind for everyone that isn't the Emperor?