I'm all for adding depth to everyone's favorite black and gold death stack, but I think that would be far too much. More cultures yes, but I don't think a hessian culture is really needed right? (sorry hessian readers lol) I like the north and south German cultures just fine in Vicky 2 and would be nice if added to CK 2. But i'm afraid of what the HRE will become with more cultures as is with the way the independence wars work in the HRE. The laws in the empire should be unique for them so that cultures like the Frankish, Czech, and Dutch rulers (ahem) don't go super nationalist just because an emperor with little control over them speaks a different language than them. The crown authority laws don't carry over well at all for Empires, instead it should only have one special authority law for the empire that is weaker than autonomous crown authority. This is not going to nerf them completely, the crown authority in it can be changed but instead each time a new member of the same dynasty of the previous kaiser is elected, the crown authority can be raised, if your dynasty keeps holding power continuously it will make you more powerful but your vassals will start to dislike you a lot if holding it for a long time, making you have to resort to sneaky tactics, keen diplomacy, and of course anti popes if you want to get the full strength of the Empire.
This will make the HRE a much more dynamic experience, since corruption is barely seen, and about 70% of your civil wars are Independence wars from some Frankish and Czech punks. I would love to actually be able to play a Holland game without an asshole kaiser raising crown authority to absolute without any vassals objecting. I can only hope that a Holy Roman Empire flavor pack will be added someday instead of trying to make the Norse playble.