I feel like it would require the possibility of Landless-esque gameplay, with powerful noble families who don't have exactly feudal territory on map, but more of a "Manor" kind of holding, like how the Merchant Republics worked in CKII.
One interesting possibility is that if you rule territory, rather than it being inheritable feudal territory, you rule as a Governor or the like under the Emperor's mandate.
Of course, if the Emperor weakens and the Noble Families become stronger, and the center cannot hold, the local power-holders should get stronger and slowly and surely obtain more power in the margins.
Which I think would be very interesting mechanics, because it's actually not that dissimilar from how Carolinglian France worked and how Feudalism in France came about.
Game really needs to be ready to deal with non-feudal govts. AFAIK Medieval Poland was an administrative kingdom, for example.