A few Ideas
-Capitals don't get destroyed, and if a revolt happens there, the nation is recreated with the 2nd or 3rd iteration
-Population, cities with higher populations will be harder to take, but are strategic in weakening a empire
-Capitals can't become cored, and have a high chance of rebelling
-Nations can be puppeted
-Larger alliances like NATO can be created
-Different terrain types
-Rebellions forming in areas far from cities
-Railroads can be built between cities
-Nations will target the capital during war, but focus on recapturing cities
-After peace encirclements are either removed or connected
-Different war doctrines for nations