You can discuss whether this isn't historic. Until the great interregnum after the end of the Hohenstaufen dynasty the crown authority in CK 2terms in the HRE was comparable to a state such as France. The interregnum really hurt the crown authority, however successful Holy Roman Emperors or king of the Romans, like Rudolf of Habsburg and Henry of Luxembourg, did start to reestablish the authority. The focus only really shifted during the reign of Charles IV of Bohemia (Luxembourg), who issued the Golden Bull, which defined the prince-electors and their almost royal privileges and shifted the focus to the own dynastic lands as base of power for the ruler.
So increasing crown authority should be possible, but it should take more time; which IMHO isn't only an issue in the HRE.
Changing the inheritance laws of the HRE is more interesting, IMO in theory it should still be possible, but you probably have to fight all your vassals in order to get their acceptance.