After TWR added many diplomacy options, I think it will be also cool to look at domestic politics overhaul.
So I think it shall target these aspects:

1) Adding political fractions in your empire from 4 era. It shall has its own window. Different type of rule system shall have different names for each type of your ruling apparatus. For example, monarchy will have council, republic will have parliament. And time from time you will enact some number of laws, which will influence your stability and etc. Also each city shall have distribution of belonging to each “party” among population. Such system was implemented in ES2. Finally, this distribution depends on your gameplay, and you have to enact different laws, and if they don’t supported by dominating parties, than it will low stability, cancel effects of laws and even lead to:

2) Revolutions (their overhaul). So now revolutions shall depend not only on stability in your cities, but also on results of wars, and how your gameplay corresponds to enacted laws and how you influenced by foreign cultures. There shall be different options to counter it, but if revolution happens, than you have to enact the type of ruling system of revolutioners, either then starts a civil war. New rebels should not be just a random IP, but have their own questline. And foreign cultures can participate in  questline and help rebels or origin culture, and get different bonuses from helping one of this side (like assimilation of rebels, or shared techs, money contribution from origin culture).
3) And finally belonging of your people to  different cultures (nationality). So now you have new population of your culture belonging, but influence from over culture, shall provide people in your empire of foreign culture(they will have the colour of that culture). Due to axes of original culture their people shall give additional bonus to FIMS, be better at defense or attack if turned into unit.  So now you will have a plenty of options, such if you enacted slavery to foreign captured cities then their people can give less stability but more production and finally lead to revolution. Or you have to manage different people to to do different FIMS due to their additional bonus


So, I think that will be enough for such dlc, share your ideas and upvote if you liked such domestic politics dlc.