Dynamic political systems, more diplomacy/espionage options.

I have said it in chat but I love the way Hearts of Iron did those. Nations would start with their real system but you could effect change over time. The systems had effects on economics and diplomatic relations.

It helps those like me that would like to play with certain countries but not their govt.

Also having options like attempting coups, assassinations, or just somewhat influencing others toward your system.

That is all