This is not new mechanics at all. Currently, you are able to send down armies to punish realms who won't do what you tell them to, and for them to give you monetary tribute, a noble must move so that a gold transfer can be made.
Yes... I think if we put a harder limit to realms size, the vassals realms will be 'created naturally'... and the oppressor realms would want others things aside gold... a few 'selected' nobles in their capital, or a few units on their army (always the first ones in suicide attacks)...
so many possibilities!
