I've actually got some ideas along this line.
I'd like to make it somewhat more nuanced than that—for instance, I'd like to make it more advantageous to grow a realm from the capital outward, rather than making it long to reach an enemy or something.
But the plans I have for implementing this are actually sooner, rather than later...I just need to be less busy at Real Work™ before they can happen.
I actually really don't like this idea. I should indeed be more difficult to keep controll over them, and you can severely limit tax income which would be realistic as there is no Lord to manage this. But letting them go rogue, neah. A realm should always be able to expand imo.