It is quite interesting and has generated lots of questions on my mind,do prepare for a long comment giving you my apologies already just lots of things coming to mind.
So on which matters can the Provincial Assemblies legislate on?I suppose the majority of powers are holded by the National Assembly and the Crown,do the duchies have control/are allowed to retain part of the taxes they gather?Can they negotiate with the Crown lower taxes?Are there Provincial Assemblies that hold more powers than the others or do they share the same limitations?
Concerning the National Assembly,How much power does it hold in contrast to the Royal family?Can the King veto legislation or outright discard it(similar to how the Duke can ignore the Provincial Assemblies a number of times)?Do the commons have the same representation there as the Dukes(considering each group sends 5 members),how does this power reflect on the National Assembly?
I understand the Provincial Assemblies are mainly composed by nobles,the commons you refer to are they a combination of wealthy merchants and representatives of the clergy?Are they elected into the Assemblies by census suffrage like they have a certain amount of wealth and influence(householders) and so they can be a part of the assembly?How many members are there on one Assembly,can the Assemblies pass reforms that extend the members limit?
Having opportunities to develop a unique playthrough is always really fun,create a Federative Monarchy that eventually can drop you when it figures that they do not longer need you or persue true Absolutism and oppress the people on your Realm with an iron fist as a true tyrant.
Also please do make the duchies have their own customs,cultures,political histories and even reference their distinct languages(sometimes there is merit creating cultures that share lots of elements like language but are still able to differentiate each other) if you are up to it,like doing it with foreign countries is great but giving we are on such a moment of Arcadia’s history you can show how the Duchies are also trying to discover their own identity,recreate their own glory or simply strive for a future of their own.
This all fits incredibly with the concepts of “nationalism” and “regionalism” if I were you beyond the Duke’s power and possible exploration I would try to reflect how complex the reality of the life on each one of the provinces can be and how people yearn for a better life,how feudal life is changing with the possible rise of the bourgeoisie,the supposed middle class once peasant’s without voice and how nowadays their struggles can affect as much and shape the land’s as the nobles.
A read to French history and perphaps how other European country reform and change feudal societies towards republics and Imperial ambitions would do you good for creating the story,as it really seems to be that key point of history where everything can happen both paths.All with the extra help of the MC of course.