Some civics keeping civs in a religious mindset is a good idea. I thought more of the Industrial era as the transitional period, as fascism and communism appear at the tail end of it anyway. The dynamic between ideologies could also change, with the industrial era focusing on the dichotomy between conservatism (?) and liberalism. Alternately it wouldn't be based on era at all, and ideologies are "founded" with a technology so that the diplomatic dynamics change accordingly.
If using Imp. Knoedel's idea of ideology spreading like religion:
At least in America, conservatism and liberalism are very similar to each other in terms of final overall philosophical foundation. Both would count under the ideology of Liberalism. The differences would be in civic choices (Social Liberalism = Republic + Egalitarianism + Public Welfare + Free Market; Conservatism = Republic + Representation/Egalitarianism + Capitalism + Free Market). Just how the differences in Socialism are civic-related (Democratic Socialism = Republic + Egalitarianism + Public Welfare + Central Planning; Leninism = Autocracy + Totalitarianism + Public Welfare + Central Planning).
You could add to America's UHV, World's Policeman, to be >50% of all cities on each continent have Liberalism.
Here are some ways that ideology could spread:
- Founding (Liberalism starts in capital of civ that first discovers Liberalism (historical given liberal ideas started in England and France pre-democracy); Socialism starts in civ that first discovers Communism; Fascism starts in civ that first discovers Fascism)
- Historical spread (Liberalism to America on spawn, Socialism to Russia in 1920, Socialism to China in 1950, Fascism to Prussia and Italy in 1940, etc.)
- Cultural spread (if a city has the ideology and culture is bleeding over into another city, whether domestic or foreign, it has a chance of spreading)
- Trade routes (so if you're liberal and don't want fascist/socialist dogma, don't sign open borders with them)
- Propaganda (can be spread by Spy unit to spread your state ideology to foreign cities)
- Immigration (for America and Canada, both religion and ideology can spread with immigrants)
Ideology should have massive influence on diplomacy in late game. In actual history, Liberalism has had control of the United Nations. However, in alt history, you could have a Socialist or Fascist UN with many different propositions. I also think UN propositions should occur more common than they currently do.