I thought a little more about ideologies and how they might be represented in the game.
1) Ideologies get a similar role to religions in earlier eras: diplomacy, (un)happiness, possibly wonders, etc. Accordingly, they have some mechanical similarities: they spread to cities to represent a receptive population, they have missionary-type units ("militants"/"propagandists"), persecutor-type units, they can be spread by Great Statesmen the same way Great Prophets do. However, some differences exist: no equivalent for temples, cathedrals, holy cities. Something like a monastery might exist (with a bonus to espionage instead of science?), or militants might be trained in some other way.
- If we want to complicate things further, the spread could also be attracted by specific buildings like factories, news corps, etc. though that might be unnecessarily granular.
- Some civics like Democracy or State Party might further influence how militants and ideological spreading happen.
2) You can't adopt a particular state ideology since that would be redundant with civics. Instead, you have a score for each ideology over how much your civics match it. So Central Planning might give you a big communist score, etc. It's possible to have a very high score in a single ideology, or moderate ones in two or three.
- Diplomatically, you receive a bonus with another civ for each ideological score that is close, and a malus for those that have a greater difference ("+2 You have liberal civics."). Instead of its leader's favorite civic, an AI might ask you to adopt new civics that get your score closer to their.
- The presence of an ideology in a city also reacts to your particular civics, through (un)happiness modifiers, possibly some other effects on buildings, etc.
- Ideally, it should be just as viable to fully commit to an ideology or be moderate between two of them.
3) As to which ideology should be included, I think we should avoid unnecessary granularity and stick to the big three (liberalism, communism/socialism, and either fascism or a broader "reaction" camp), with older more traditional ideologies like conservative monarchism simply being represented by the absence of those three, with the appropriate diplomacy modifiers.
- Reaction or fascism? I'm inclined toward the former name because it's a much broader umbrella term across modern history, but it's a bit harder to pinpoint what tech should start it. It could be reaction as an ideology, but still keep fascist dynamic names and particular benefits if you adopt Totalitarianism.
- Each ideology could have some minor aspects unique to them. As an example, I'm thinking in particular that reaction could have lesser positive diplomatic modifiers with civs that do not share the same religion/secularism civic, and positive ones with civs that have the same religion and are reactionary or merely conservative.
EDIT: Thinking back on it, there may be a fourth ideology that did have enough geopolitical impact to be worth including: third worldism/decolonization/anti-imperialism/whatever name isn't too unwieldy. That one might be a bit less dependant on civics (though obviously some like Colonialism, Tributaries, etc. would still be impactful) and instead get entirely different modifiers:
- Having vassals vs being a vassal,
- Having territory outside of your historical one and especially inside another civ's core, vs the opposite, having another civ in your core,
- Diplomatic tributes of resources, etc.
- Maybe something to do with lagging in techs?
That one might produce a more organic shrinking of the huge colonial empires by putting pressures on them through unhappy population and diplomatic penalties against the smaller civs.