The idea would be to allow realms to be vassals of other realms, but lowering the maximum size any of those can achieve by themselves only. Being realm B's vassal would cost realm A a % of its total tax income, its ruler income, or some other form of revenue. The point of this is to create a hierarchy between the small feuds in where several are in the less comfortable position of inferiority regarding the dominating realm, which should not be in itself excessively stronger than the individual dominated ones. But being careful not to allow the mechanically-imposed tribute to be so taxing as to effectively making eventual opposition of the center of power impossible.
This is almost what the Duchy mechanics does. Dukes send x% of their revenues to the overall rulers; however Dukes retain the capacity to secede, therefore retaining some of their autonomy.
The difference is that if Duchies were vassal mini-realms, they would be able to fight each other. Now, I like this: I have always advocated for a form of intra-realms warfare. I know many people don't like this. Maybe attacking the question the other way around would work better? Don't implement intra-realm warfare, but make it easier to create HRE-like entities above the level of realms?
