So far in my games I have seen civics only have a very small impact on diplomacy, whereas in real life the government choices and the ideologies they represented was (along with religion) one of the main forces splitting countries into factions. My proposal is to have diplomatic modifiers based on some individual civics and the ideologies they clash with. I have drawn up a list of what I think best represents these differences. I think this will make players think carefully before making key decisions about their civics as well as encourage them to force other civs to share their ideologies. It is also more realistic.
Hereditary Rule
Representation & Universal Suffrage = -2 "we are afraid your revolutionary ideas will end our reign as Monarchs"
Representation & US
Representation & US = +1 "You are a supporter of democracy"
Police State = -2 "You are an oppressive dictator"
Emancipation
Slavery = -2 "You are a slave trader"
Free Market
Mercantalism = -1 "You are suppressing free trade"
Free market = +2 "You have embraced free trade"
State Property = -3 "You are a communist!"
State Property
Mercantalism = -1 "We dislike your imperialistic policies"
Free Market = -3 "You are a capitalist pig!"
State Property = +2 "We will continue to support you, our Comrade"
Theocracy + OR = -2 "You are too religious, it is the opium of the masses"
Theocracy
Free Religion = -3 "Your country is filled with atheism, heresy and paganism of the worst kind"
State Property* = -3 "you spread atheism like a plague, we fear for the morality of your populace!"
*If they also have Free Religion
Hereditary Rule
Representation & Universal Suffrage = -2 "we are afraid your revolutionary ideas will end our reign as Monarchs"
Representation & US
Representation & US = +1 "You are a supporter of democracy"
Police State = -2 "You are an oppressive dictator"
Emancipation
Slavery = -2 "You are a slave trader"
Free Market
Mercantalism = -1 "You are suppressing free trade"
Free market = +2 "You have embraced free trade"
State Property = -3 "You are a communist!"
State Property
Mercantalism = -1 "We dislike your imperialistic policies"
Free Market = -3 "You are a capitalist pig!"
State Property = +2 "We will continue to support you, our Comrade"
Theocracy + OR = -2 "You are too religious, it is the opium of the masses"
Theocracy
Free Religion = -3 "Your country is filled with atheism, heresy and paganism of the worst kind"
State Property* = -3 "you spread atheism like a plague, we fear for the morality of your populace!"
*If they also have Free Religion