Enlightenment is the main resource used to research technologies, pass reforms, buy stability, sway diplomacy, and many other factors. Ultimately, it buys you victory directly, and it ties into the espionage level of gameplay.
Enlightenment is generated by a set of buildings, each with its own bonuses, but these buildings produce but a little trickle. More significant are the Gentlemen characters spawned by your Universities. Gentlemen represent the great gentlemen-scholars of the era, who often wore many hats. These Gentlemen have a Prestige score that is their power level (think veterancy) and act as both intelligence and counterintelligence. They have various missions they can undertake and their effectiveness depends on their network. In real life there was this thing called the Republic of Letters where the Enlightenment philosophers were part of a big circuit of correspondence (later this turned into the concept of an academic journal), and suffice it to say, they were girly drama fags. Gentlemen can enter into a Philosophical Friendship or a Philosophical Rivalry that set rules about how Enlightenment can be shared across nations and how Prestige is added up between networks to decide the odds of mission success. Rivals, basically, a breaks in a network and open up more aggressive actions, while Friends let you tap into more of a network.
Ex: David Hume has a Rivalry with Jean-Jacques Rousseau. Rousseau can initiate "Pamphlet War" on Hume.
Ex: Marquis de Lafayette has a Friendship with Thomas Jefferson. France gets an Enlightenment bonus, through Lafayette, for all the Enlightenment Jefferson produces; America gets an Enlightenment bonus, through Jefferson, for all the Enlightenment Lafayette produces.
These bonuses grow the bigger the network gets; three mutual Friends are more powerful than one Philosopher being Friends with just two. Friendship always pays off - cooperation is always productive - but that doesn't mean it's equally profitable. Sometimes cutting off a rival power is worth it, which means antagonizing their Gentlemen.
Enlightenment buildings usually have passive effects but also active abilities that have to be expressed through a Gentleman; the Gentleman is like the roving agent that allows the building to extend its reach. Some abilities and buildings I'd thought of were:
University = Produces Gentlemen; if one dies, a new one is generated to replace them; your basic infrastructure in the game of ideas; can house one and only one Gentleman's Cabinet of Curiosities as a Museum; enables "Compose Magnum Opus"
Observatory = Assists in naval navigation, enables "Steal Navigational Charts" and "Gentleman Pirate"
Printing House = Enlightenment mill, enables "Pamphlet War"
Opera House = Raises Publick Admiration, enables "Political Satire"
Academy = Raises Morale for fleets, improves gunning, boarding and land warfare ability, enables "Duel" maybe
Embassy = Buildable only in rival Metropoles, acts as a Coffeehouse/Salon that also enables "Slander Nation" and "Proxy Privateering"
Tavern = Built offensively in other nations' Harbors, enables "Sabotage" "Assassination," passively reveals ships that were last stationed there, local resource stockpiles/build orders and trade convoys that run through the Harbor (where they're from, where they're going)
Black Chamber = Passively reduces success chances of hostile Tavern actions and increases friendly Tavern success chances; actively, enables "Counterintelligence" and "Deporation"
Coffeehouse/Salon = Built offensively in other nations' Harbors, enables "Seek Friendship" and "Court Intrigue"
Conservatory = Raises Publick Admiration by a lot
Cathedral = Raises Publick Terror, enables "Moral Panick" and "Holy War"
Mission = Only Enlightenment building available in a Feitoria, acts as a cheap, low effort way to print some Enlightenment
Stock Exchange = Raises Gold revenues from trade, enables "Speculative Bubble"
Gentlemen abilities:
Personal
Seek Friendship = Establish a Friendship
Insult = Establish a Rivalry
Duel/Seek Satisfaction = The way you actually kill Gentlemen: a fight between Rivals. If both survive, there's a mutual gain of Prestige and the Rivalry ends.
Pamphlet War = A gamble on raising your Gentleman's prestige, reducing the target's, and cashing in a bonus of Enlightenment.
Political Satire = Reduce Publick Admiration in the target Harbor
Court Intrigue = Force a Friendship to dissolve or a Rivalry to start between two other Gentlemen (used to create ruptures, offensively, in other nations' networks)
Moral Panick = Nuclear option for isolating your network: downgrade all relationships a step between your Gentlemen and those of the target nation (Friendship to neutral, neutral to Rival)
Compose Magnum Opus = Limited to one per Gentleman, makes a big bonus of Enlightenment based on the current Prestige level. You only want to cash this in when needed or when you doubt you can get much more out of developing your Gentleman.
Retire Gentleman = A Gentleman will eventually (since the game represents about 200 years of history) get old anyways and have a probability of withdrawing/dying that escalates dramatically. Once they're doomed, their productivity at anything other than writing a Magnum Opus falls. You can manually retire them to get your replacement up and running sooner.
Gentleman Pirate = Enables use of Gentleman as a Captain and of Captain as a Gentleman
Matters of State
Counterintelligence = Reveals and destroys all Taverns in the Harbor
Deportation = Boots a target Gentleman from your country and bans them from reentering for a while
Steal Navigational Charts = Reveals a Harbor or section of sea's Navigational Charts that you lack
Slander Nation = Damages AI relationships; against human players, causes illusions of aggressive military build-up near them; is targeted towards the nation you're influencing against any other nation
Proxy Privateering = Used in a friendly power against a nation that they are hostile towards but not at war with, allows you to recruit Privateers from their merchant marine/build them from their harbors. (This is inspired directly by Benjamin Franklin in Paris.)
Expel Ambassadors = Convince host nation to expel another nation's Embassy
Holy War = Gain military bonuses against the target nation, raise Publick Terror in your own nation (war hysteria)
Speculative Bubble = Cash in a ton of Gold and spike Publick Admiration, but the Treasury empties out, Gold prices skyrocket a while later and Publick Admiration tumbles; basically, you can use this as a "selling the farm" ploy to raise a ton of money, but you're obligated to spend it NOW NOW NOW and finish what you're doing because you will be in even more severe financial and political distress later.
Adventuring
Gentlemen are not units that can just be killed, but they do move around the map. You can have one hitchhike, basically, on any ship, which is how you infiltrate hostile nations (in this era it is, to my knowledge, totally realistic for foreign nationals of hostile powers to be let in to a country and allowed to roam freely; a dedicated Assassination, Duel or Deportation is needed to get rid of them). On your own ships, you can use Gentlemen as explorers.
On an unsettled island, a Navigational Expedition reveals the island and allows you to colonize. A Natural Philosophy Expedition can be used to print up large amounts of Enlightenment, but it can be risky to the Gentleman's health. Think William Dampier and Charles Darwin as examples of scholars going overseas. Expeditions can yield Curiosities with little bonuses that accompany the Gentleman and can be bequeathed to a University on their death. (These would, I guess, be like your Relics in Age of Kings.)
Captains can form Rivalries with other Captains they have fought and can fight Duels too; a Captain in a Harbor with a Gentleman is one way for the two systems to intersect, as is converting a Gentleman or Captain into a Gentleman Pirate.
Tavern abilities
Sabotage = Locally sabotage a building, fort or even ship in the harbor (playing this reveals that there is a Tavern there)
Assassination = Way to murder Gentlemen without risking your own Gentlemen in a Duel (playing this reveal that there is a Tavern there)
POLITICAL SYSTEM
The political model is Machiavellian: it is better to be feared than loved, but better still to be both, and you must never be hated. I decompose this Publick Opinion into Publick Admiration and Publick Terror. Admiration is fickle; when things are going well your population is happy and supports you, but it is lost very easily. Terror is a virtue of moderation: none of it and nobody respects you. Too much and they hate you and are willing to do anything, bear any cost, to get rid of you. In moderation, they fear you but don't hate you, and so are paralyzed.
Feared and loved = Your nation actively volunteers its enthusiastic, whole-hearted support.
Feared, not loved = Your nation is obedient but goes no step further: no revolts, but no bonuses. The moment local forces seem weak you get explosions of rebellion on your islands.
Loved, not feared = Your nation is fickle. As long as what you are doing matches what they want, you get bonuses. The moment you go against that, or things turn against you (like a military defeat), "love" disappears.
Neither loved nor feared = Quite bad, a powder keg of a state.
Hated = Nothing else matters. High Terror destroys Admiration, so you won't have high Admiration and Terror at the same time anyways, and you get constant chaos.
For the most part, Publick Admiration comes from giving your people bread and circuses, bread being integration into your mercantile network, circuses being local entertainment/propaganda buildings and triumphs at sea (Captains defeating rivals, big naval victories, successful raids and plunder) and in the intellectual sphere (winning pamphlet wars, composing magnum opuses, etc.). Publick Terror comes from garrisons, forts, Prison Hulks (think a moving Terror generator), Gibbets, and such. Admiration is mostly a feedback loop of playing well feeding more success and it can be broken suddenly and shockingly. Terror is more a matter of costly resource expenditure (things that do not further society's advancement in any way).
Slaves only really respond to Terror, being... you know, Slaves. On a sugar island things are simple.
COLONIAL EXPANSION
Encroaching on a "virgin" (settled by natives) land never makes the natives happy. Think: very rebellious island with lots of guerillas attacking stuff by default. There are various ways to deal with this, essentially Subjugation, Cooperation or Expulsion, Spanish, French and British respectively. Subjugation depends on military beatdowns and leaves behind a larger population but a less loyal one. Cooperation depends on economic integration and leaves less flexibility as to how the island can be developed; it is generally a cheap and easy option that gimps your growth. Expulsion is military-lead, like Subjugation, but sacrifices the population in exchange for building a more loyal and developed colony from scratch with more growth potential. This might be represented by the natives taking up land until they're removed.