Even without applying all of this, what would be much easier to change is the current realm size penalty bonus into a duchy size bonus.
The more regions are part of the duchy, from a certain amount onward(like 4 or 5), the bigger the realm penalty.
Realms that ignore duchies and maintain a single large duchy get more penalized then realms with many smaller duchies.
This would already make dukes more important, increase their role and increase their numbers.
This wont effect the core of the game radically as the ideas suggested earlier.
Allthough i do really like the ideas of duchal capitals.
