Some ideas for the different options from the main elements of your society:
LEGITIMACY
- Theocracy, from each enabled religious doctrine you get a Commandment that must be keeped to unlock an additional bonus, however if any Commandment is not keeped it will show a turn timer to receive loyalty penalties from it. Most of their positive effects are directed to the population of your official religion, including a huge boost to loyalty.
- Monarchy, every certain number of turns you can choose a Consort from a list of Royal Houses that at the beginning are one from each owned city, plus one from every friendly civ that is also a Monarchy. Each time is generated a new set of Consorts with random bonuses, then the selected homecity would gain a powerful loyalty boost plus a title, up to three levels (1-Barony, 2-County, and 3-Duchy) for a total of 4 bonuses. Then if you keep selecting the same House you would need to grant them others cities but from here the more you keep doing it the bigger the chance of a revolt from all the cities of other houses (based on % of cities). Meanwhile Consorts from foreign Houses replace the basal loyalty bonus with a diplomatic bonus.
- Republic, here every kind of class, heritage or belief whose denizens represent at least 5% of your empire's total population would gain a Senator representative that provides a loyalty bonus. Also you can pick three Senators (one of each type) to provide empire wide bonuses. If the percentage for some representation is lost it raises an alert and its effect expires in 3 turns.
AUTHORITY
- Autocracy, players can make Decrees, powerful bonuses for a specific topic to select, but only one can be in effect at once. Even if a decree can be replaced by a new one any turn the player wants, when enacted it causes a generalized loyalty malus for X number of turns, so two consecutive decrees before cool down would accumulate their negative values.
This option also has a default loyalty penalty to any population with a value below average.
- Oligarchy, depending what combination of legitimacy and/or ideology the player has, certain kinds of citizens would be assigned as elites, this could be for example the clerics for Theocracy, the warriors for Monarchy, the traders for Capitalism, etc. Then every elite class would have their Pacts, a set of bonuses that can be unlocked by their level of loyalty, so the happier they are the better bonuses they provide. But to secure this you would need to invest in favors and privileges that would mean spending yields and sometimes affect the other kinds of citizens.
- Democracy, every certain number of turns Elections will be held. When an election is held every kind of citizen whose average loyalty is at least X level would provide their own special bonus. Here the individual bonuses are more modest but there are way more identitarian groups, so the balance of the interests of our whole population would boost every aspect of our empire.