So, I have a crazy idea on how to implement internal politics.
Basically, I'd divide power inside the state into factions, arbitrarily split into five in my example. The more power a faction has, the more positive and negative effects it brings to play. However, if the faction is completely powerless, they still cause negative effects, portraying the society lacking that part entirely... Around 20% power, the faction would be neutral in their effect. (See the faction list below.) In addition to the factions, there would be government forms, which have similar effects, plus stack the deck to the benefit of certain factions. This way you could, for example, have a Democracy really ran by Corporations. (See the government form list below.)
In addition to these two things, I'd add corruption, which would be a percentage of everything (resources, research points etc) wasted. It is stolen, sold on black market, lost due to inefficiency etc. Hinterlands of an empire should suffer form increased corruption until they are brought fully inside the empire somehow.
Government forms:
Dictatorship/Monarchy (+military bonus, +intel bonus, -unrest more common, -more corruption, favors Aristocratic/Militarist factions)
Democracy (+less unrest, +minor tech bonus, +minor economic boost, -intel penalty, -military penalty, favors Corporate/Humanist factions)
Oligarchy (+economic boost, +minor intel bonus, -minor unrest malus, -more corruption, favors Aristocratic/Corporate factions)
Bureaucracy (+a little less unrest, +tech bonus, -economic malus, -military penalty, favors Technocratic/Humanist factions)
Thought Control - requires research to activate (+almost no unrest, +huge intel bonus, +military bonus, -large economic malus due to upkeep, -substantial tech malus due to groupthink, favors Militaristic/Aristocratic factions)
Instant Democracy - requires research to activate (+less unrest, +less corruption, +tech bonus, -economic malus due to upkeep, -military malus, favors Humanist/Technocratic factions)
Corporate Federation - requires research to activate (+intel, +economic bonus, +small tech bonus, -more corruption, favors Aristocratic/Corporate factions)
Factions:
Aristocratic/Oligarchic/Caste/Elite (+theoretical science, +intel, -increased corruption due to nepotism, -increased unrest due to class society)
Corporate/Capitalistic/Merchant/Trader/Monetary (+economic boost, +applied science, +excels in quick exploitation of planets, +corporate colonies, -theoretical science, -more unrest due to poverty, -more corruption in the long run)
Technocratic/Scientific/Transhumanist/Futurist (+large tech bonus, +small unrest reduction, -economic malus, -intel malus due to academic freedom)
Humanist/Democratic/Unitarian/Altruist (+corruption reduction due to transparency, +unrest reduction, +faster assimilation of other cultures,
-economic malus, -intel malus, -military malus)
Militarist/Xenophobic/Patriotic/Fanatic (+military bonus, +intel bonus, -small corruption increase, -tech penalty due to authoritarianism, -slower assimilation of other cultures)
How does this work?
I'd have the game fire up events, which change the faction power in the society. These would be a random element, possibly more common in democracies and less common in authoritarian regimes. In addition, there should come up choices for the player, such as how to organize a new colony. Letting a colonial trade company run it would increase Corporate power, making it a military outpost would increase Militarist power etc. Sometimes you could get direct suggestions form a faction with a prize attached, like Corporate offering you XYZ resources if you conquer planet Y or make a trade deal with faction Z. These, in turn, would increase the faction power.
I'd like to see certain factions (Aristocratic, for example) be mainly negative due to their faction efforts, but able to offer so good instant rewards that player will grant them more power. This way, the player would slowly damage himself by taking short term gains. This, I believe, would nicely portray the slide of empires into corruption and decadence. Attempts to renew the situation should include policy changes (like changing the colony types away from noble fief into something else), which would cause the worst turmoil the stronger the faction is. It should even have a chance of firing up a civil war.
Likewise, there could be events that can fire based on faction power. So if the Militarists/Aristocrats run the show, they could turn a Democracy into a Dictatorship, basically forcing through government type change without consulting the player. Too strong factions would basically try to usurp the things player can do for their own ends.
If you ever added stuff like system governors, admirals and so forth, they should each belong to a certain faction. This would make civil wars etc easier to represent, I imagine, by having a Militarist admiral take his fleet to the Militarist side etc. I could see too strong factions pushing through governors/admirals/whatever that are actually very shitty, but belong to the right faction. Likewise, the players might need to consider whether choosing a very skilled governor that belongs to a faction the player doesn't want to boost or to choose a worse governor that belongs to a faction he'd like to grow stronger.
Likewise, I imagine some specie (like an insectoid hivemind or a robot collective ruled by an AI) could have a special ability of simply switching between factions as they want every 50 turns or something, to portray their lack of inner politics and the ability to focus completely on something as a species.