I am disappointed at the negative feedback, but nevertheless...
You don't only have three choices, you have many, any type is possible. If you are inbetween Democratic and Authoritarian, it would be something like the English monarchy with the parliament. I you were inbetween Socialist and Authoritarian, it would be something like Stalinist Russia.
You would have different government positions which, depending on what type of goverment is currently in power, you may be able to place, or not. Such as: Head of State (Authoritarian), Head of Government (President/Prime Minister), Foreign Minister, Minister of Intelligence (effecting Espionage), Cheif of Staff (Military), etc. Each would have their own ideology, and will contribute towards drift in a certain direction.
Each government type would allow different elections to take place, or none. Depending on the amount of control allowed to you. Civics would still survive, in the form of different laws. Such as: Open society, limited restrictions, repression, and a totalitarian system. All would have different effects like increased dissent, greater production, or more control. Conscription laws would help build your army, but in democratic societies, my cause dissent. Economic laws would allow more or less control of your economy to the people, or to the government. Press laws would allow for more control of the opinions of your populace towards a certain enemy, or potential enemy, but may cause more dissent.
Diplomacy is also affected by this system. Democratic nations are more inclined to help another democratic nation in need than ones of another ideology. Socialist nations may trade resources and supplies with little to no cost with another socialist nation. Authoritarian nations may call other authoritarian nations to help them in a war, or they may declare a 'limited war' so that they can invade another country without missing out on any spoils of war to any of their allies that would otherwise join in.
Revolt may also play into this, the farther from your ideology the country you conquered, the more likely a certain city will revolt. To reduce this, you may change your hold on their lands:
Total Exploitation: full control, using extreme repression to get the maximum amount of production possible. This causes the most dissent.
Full Occupation: Military takes total control, it is a little less repressive, and causes a little bit less dissent.
Military Government: The highest position is filled by a military commander, while most of the rest of the civil administration is left in place, causing much less dissent, but a lower amount of production.
Collaboration Government: The former government is still in place, but they are totally under your control. This lowers the dissent considerably, but provides the least amount of production.
There is also another option, which is to create a 'puppet state', that is the 'kindest' form of occupation, and will be seen in a better light in diplomatic relations. The puppet state would immediatly have the same ideology and diplomatic relations as you, if you declare war, it does too. If you place an embargo, it does too.
Some of this I like, and some I dislike.
I will start with what I dislike:
1. You put far too much emphasis on ideology. Ideology is but one part of government; at least as important are economics (extremely important), culture (as distinct from ideology), demographics, and so on.
2. Essentialism on the part of ideologies. In particular, the Democratic Peace Theory, which you allude to, has not been definitively proven, and the observed effect of "modern liberal democracies don't generally fight one another" can be adequately addressed by other social factors: it isn't necessarily true that democracies don't fight one another, but in a democracy popular opinion matters, and in stable liberal democracies popular opinion generally holds sympathy for other liberal democracies. Younger liberal democracies and illiberal democracies, however, are historically actually more likely to go to war, as they tend to have a surge of national pride and popular empowerment that most easily lends itself to lashing out at neighbors.
Things I like:
Levels of occupation. Interesting, though details would have to be hashed out.
Things I think are missing:
A clear system of determinants of social (and thus governmental) change. I think I can handle that, tho.
Without further ado:
The structure of the governmental and social system rest on the character of your people, which in turn is defined by the conditions within your empire. The conditions are both material (economics, demographics, geography, etc.) and non-material (cultural/religious elements, mostly). These I term the
determinants of your society.
The determinants determine the social character of the people: their social prejudices (or lack thereof), their political preferences, and their ideological convictions. These in turn shape the sort of government you rule over, but not without input from you. You get to make policy decisions that affect the determinants, meaning that the genius of the people represents your own particular spirit.
In more detail, the determinants could be any number of the following:
Material determinants
1. Land area: How big your empire is, counting land and fresh water lakes only.
2. Population: For purposes of population, Towns are equivalent to a city of size 1, Villages 0.75, Hamlets 0.5, and cottages and all other improvements (save Roads and Railroads) 0.25. Rather unrealistic, but it works well with most of the economic models I've seen.
3. Demographic factors: Age, health, and so on could be modeled and have an effect.
4. Rural/Urban distribution of population
5. GDP/intensity of development: How much you extract from the environment, or in other words, how many things your workers built and your citizens are harvesting, or are being directly produced in the cities (e.g. with Markets, Forges, Banks, Factories, and so on).
6. Rural/Urban distribution of wealth generation: The proportions of where your GDP is being produced. Hammers and food count towards GDP as well as commerce.
7. Economic openness: How much (trade route) trade you do with other civilizations, both in how much gold you get from cross-border trade and how many civilizations you have trade links with.
8. Technology: How much you use which technologies to extract resources.
9. Proximity: How close your neighbors are to your average citizen.
10. Material History: Have your neighbors been constantly attacking you, or have they been nice? Does trade with them favor you or them? Have you been the one doing the conquering?
Non-material determinants
11. Cultural openness: How much exposure your average citizen has to foreign culture, including how many and how widespread foreign religions are in your empire.
12. Cultural dominance: Does your culture dominate your neighbors or do they dominate yours? Measured by the prevalence of foreign religions vs. the prevalence of your religions abroad, as well as the size of the foreign population in your empire vs. the size of your population abroad.
13. Ideas: Perhaps coming from Great People, definitely from the ideological technologies. If you are the first civ to discover Liberalism, that would probably have an effect on your people.
These determinants are linked to various sliding scales, reflecting the genius of the people (to use James Madison's phrase); I'd go into that, but I'm afraid I have to go.