I think that the HRE should only have their Southern provinces. The northen ones should be rebel or a new nonplayable faction. Ergo the Healeanistic League or whatever its called. The league of North German trading cities that dominated North sea and Baltic trade for a long time. They should also be highly restive when conquered because of the encroachment of Protestantism. So for every fifty years you get a lower loyalty if those provinces are conquered because there are more and more Protestants living there.