World Congress and Diplomatic Victory
The World Congress should be a place where civilizations bargain for diplomatic influence, it should rely less on city-states and more on negotiations between civilizations. The range of resolutions should be extended in order to have more options going from embargos to international laws, for example banning war without casus belli, and projects.
The Diplomatic Victory should be about creating a network of alliances and more essentially preserving a somehow world peace through conflict mediation.
Corporations/Market and Economic Victory
The Corporations should be a mixture of what they’re in Civilization IV and what religion in Civilization V looks like. The first corporations should be available during the Renaissance and should be orientated toward trade and luxury/food management, like the British/Dutch East India Company, then when you reach the Industrial era you start unlocking more specialized corporations, such as financial, mining, or entertainment. If used well they should significantly change the mid/end-game experience by giving ways to expand influence through the economy.
The Market should be the place where you can trade resources exploited by the corporations, and it should also be the place where you can acquire shares of other civilizations corporations. If a certain resource becomes rarer or abundant on the Market it might harm the civilizations/corporations relying on such resources, alongside of that you can also speculate on resources or corporations, which increases income generated by the resource or corporation, however if you hit a certain threshold you might start an economic crisis affecting other resources/corporations.
The Economic Victory should be about having the most competitive economy and about being the best at handling economic warfare.
Cultural Influence and Migration
Cultural Influence should be similar with the way it was handled in Civilization V and Migration should be a mixture of how migration works in Stellaris and how the religious followers work.
If you’ve a civilian open border treaty with another civilization, the population might want to move to a more attractive city from another civilization (according to a set of conditions), meaning you might lose or gain population. However if a foreign city has more population coming from your civilization than their own, you’ll have more cultural influence on this city and you also be able to use a casus belli to conquer this city.
Cultural Influence works like in Civilization V, meaning if you’ve more influence over a civilization the population of this civilization will more likely rise if government/religion are too much apart from the ones of the cultural dominant, same in case of war.
Revolts and Civil War
They should enhance the way your population take arms against you in order to make them a real threat. The population should rise according to the level of cultural influence (movement to join another civilization or being independent), their desire to regain their freedom if they’re conquered, their desire for independence if they’re far from the capital and if their cities has a bad economy, and if they want you to change your government, adopt another religion, or stop a war (in those situations you’ll not lose cities). Outside civilizations can use covert operation to help the revolutionaries and destabilize civilizations.
If several cities are taken by revolutionaries they’ll become more complex city-states and less complex civilization, you can make peace with them and allied with them, they have their own military, the casus belli are applied to them, but don’t have envoys and don’t compete for victory.
Obviously
New technologies/civics, new policies, new diplomatic options, such as “redesign borders” during peace deal where you can take tiles without taking cities, new civilizations and city-states, new wonders and units.
