During the Napoleanic wars practically all of continental Europe became protectorates of France because it was a powerful terrifying land grabbing nation. They all sent armies to assist Napoleon in crushing Russia

Then they all turned on France at the first sign of weakness and allied together to defeat him

So rather than puny rouge nations fighting you. The game should be able to form alliances against you more easily. It would be cool if a diplomatic message said nations X,Y and Z demand that you cease hostilities or maybe surrender province A, If you do not comply it will be war

Then if you go to war all those nations and their economies will be geared towards destroying you. They will make peace with all of their other enemies or put those conflicts on the back burner and also try to pull other nations into the war against or at least stop trading with you which should be easy because of your constant expansionist threat.