I have a new suggestion: in addition to the revolt/rebellion script of reemerging factions, I think that certain regions should also be allowed to have referendums to leave your empire and become rebel states again.
This applies to England and Wales region, and Scotland for that matter (why not throw Northern Ireland into the mix, since it is also a territory you can conquer), as well as Catalonia, Basque country, Brittany (that's in France), Corsica, Occitania, Bavaria, South Tyrol, Sardinia, Sicily, Veneto, Northern Epirus in Greece, Flanders and Wallonia in Belgium, Cyprus (the northern part at least), Kosovo, Crimea, Chechnya, and the Caucasus!
In essence, much of the campaign map.
And each time you get a "Referendum Successful" message on the campaign map, this is the picture used in the pop-up scroll:
However, to counteract this, the territory that just rebelled against you gets a -1000 mnai economic penalty each turn that hurts its ability to recruit more troops and build new infrastructure.