How about peace treaties as an active diplomatic action. Where it is possible for something to ask to abandon the national territory or make yourself neutral (separate and international). At international there will be a debuff on diplomacy and only countries with communism or fascism will be able to declare war.
What about federations and confederations? You can implement confederations on the basis of HRE, and the federations as autonomies and the consequences of an alliance with another state.
How about new types of puppets, such as colonies and dominions. It could also be done that each puppet can achieve some kind of "independence" through diplomacy.