When you are Emperor and hold the Kingdom of Cyrodiil, your Cyrodiilic vassals tend to create the Oblivion counties corresponding kingdom titles (Kings of Anvil, Bruma, Kvatch, etc). I find this particularly unimerssive. Two possible solutions would be forbidding the creation of titular titles if the de jure high kindom exists or, like the GOT mod does for the Lord Paramounts, having a mechanic that changes these kings into nominal "counts", without crowns and without being called kings. The second option would be useful if you want to organize your vassals into super dukes to mimic the game's de jure setup.