Would it be possible to add some minor kingdom titular titles that you can create when you own a High Kingdom as an emperor? Currently High Kingdoms are empire sized, and whenever you start growing it becomes a real nuisance not being able to delegate to relatively small vassals. Just to give a quick example, I have a current game as the Aldmeri Dominion and, after conquering Cyrodiil, I decided to hand it over to a vassal duke (making him a vassal High King). His vassal limit was 30 (with 12 diplomacy), yet he had 40 vassals, causing severe problems to the administration of the realm.
I think it would be far more useful, and not gamebreaking at all, to be able to use some of those titular titles that are destroyed automatically when you create a High Kingdom. Otherwise the only way to effectively rule Tamriel using vassal kings is to rule from Cyrodiil and let other "minor" High Kingdoms (Alinor, Elsweyr, Argonia) be ruled by a vassal. Being an emperor with good demesne laws allows you to handle the massive amount of vassals in the Cyrodiil region, and I just want to rule Tamriel from Alinor or Mournhold : P