You make federalization a tech group, and then make all states a colonization area. When something is colonized and then cored, a colonization nation is formed. Then an event triggers which brings the nation under a PU. And we also need to make to where republics can have PU's.
The federal government also has a modifier which lower liberty desire for subjects and extra force limit from subjects. And then we can create an event chain which corresponds with the era, which slowly erodes the rights of states of the federation, giving the nation state more force limit and tribute and the individual states less force limit, but more liberty desire which erodes over time. In this sense it will then work similarly to Japan, where the state of the nation state can band together and start a civil war.
The federal government also has a modifier which lower liberty desire for subjects and extra force limit from subjects. And then we can create an event chain which corresponds with the era, which slowly erodes the rights of states of the federation, giving the nation state more force limit and tribute and the individual states less force limit, but more liberty desire which erodes over time. In this sense it will then work similarly to Japan, where the state of the nation state can band together and start a civil war.