Post Apocalyptic Era

Introduction

An alternative to the contemporary era if the pollution threshold gets too bad, can be toggled on instead of the game ending on reaching the pollution threshold. The cultures remain the same for people who have already reached the contemporary era, and anyone who hasn't is automatically brought up and has to decide from the list of remaining cultures in one turn. This era will not trigger if the pollution threshold is reached before any player has entered the contemporary era.

Rebels are stronger, food and production penalties are severe, the world congress is disbanded, all independent states go into decline and certain luxury resources lose their bonuses. All cultures receive only one bonus cultural trait for the era, and the rest is identical to their contemporary era counterparts. All trade between non-allied nations is stopped, and must be restarted manually and for an increased cost. Airports and train stations enter an "abandoned" state and need to be repaired before being able to be used again. This is at first expensive but can be made cheaper after advancing in the new technology tree.

As to why this would in any way be fun or add anything to gameplay:

Every player remaining in this era starts exponentially bleeding era score. So, the player with higher era score will lose more era score every turn than the ones with less. This will be a set amount decided in the beginning of the era and can be changed under the Utopian and Dystopian civics tree. This bleed mechanic should not be too harsh and should be fairly balanced for all players, but the general idea is "the bigger they are, the harder they fall".

CIVICS

The civics tree is completely replaced, and now adds a new axis (haven't though of a cool name, leave that to the devs), with the extremes being "Dystopia", and "Utopia". These axes can be achieved by adopting the new policies, local and world events, and actions taken by the player. 

Dystopia

This path is for players who want to focus on rebuilding their nation and conquer the world of ashes. These decisions and events allow players to choose between military, production and stability bonuses. If the world is majority Dystopian, their units incur minor combat penalties against Utopian units, but conquered Utopian cities gain massive production, defense and stability bonuses. These bonuses are reversed if there are more Utopian empires than Dystopian. These bonuses/penalties are nullified against other Dystopian empires and Utopian cities which have converted to a Dystopian cultural affinity.  War score required to capture cities is also reduced, along with other benefits for an aggressive military style, regardless of the other player's affinity.

Dystopian empires will continue to lose era score throughout the game, but can "steal" era score from other players by killing units, conquering cities, pillaging and other hostile actions. This "steal" affinity gets stronger as the player reaches the extreme of the ideology axis. Mainly meant for a more chaotic and militaristic world conquest game.

Utopia

This is meant for the players who want to reduce the penalties of the era and help rebuild the entire world in a more peaceful way. Their policies and events allow players to choose between bonuses in science, food, and stability. These players also passively gain leverage and can use this leverage to sway other empires into the Utopian way. Utopian cultures can no longer spread their influence to other Utopian cultures, but gain an increased conversion rate against Dystopian nations which is again increased if they are at war with them. The Utopian ideology axis allows you to gradually reduce the era score bleed penalty, completely nullifying it once you reach the end of the ideology axis (Note: You are still prone to losing era score if a Dystopian empire steals it from you). 

Dystopian cities which have been converted to Utopian affinities will incur massive stability and production penalties. Utopian empires gain a combat bonus against Dystopian units, but also incur a unit production penalty. If there are more Utopias than Dystopias, the world congress can be reformed (under a global event where at least 66% of the Utopian candidates choose to do so. This event does not trigger until later in the era, to prevent snowballing of Utopian cultures). Losing cities as a Utopia will lead to all cities in the empire facing a stability penalty for 10 turns, which stacks the more cities you lose. This playstyle is meant more for players on the diplomatic and peaceful side.

TECH

The contemporary era tech tree is replaced by a new, post-apocalyptic tree. Any previous technologies researched in the contemporary era remain, excluding combat units, but all research invested in ongoing technologies is nullified. All empires not more than a single era behind contemporary are brought up to this era. Empires not yet in the Industrial era gain massive research bonuses leading up to this new era. 

This new tree provides new units, new districts, and new bonuses. Districts may include refugee camps (+Influence, -Stability - Utopian), or labor camps (+Production, -Stability - Dystopian), for example. Units will be similar to their Contemporary counterparts, but with slight changes and most vehicles being less effective. However, Emblematic Units and Quarters remain the same. The bonuses from the tech tree are mainly to mitigate or nullify the Food, Production, Stability and other penalties from the commencement of the era, along with other effects such as the cost to restart airports and railway stations. Reaching the end of the tech tree can be both a victory condition (if enabled) or a way to constantly gain more era score. 

END GAME

The game can then end in multiple ways:

1. Dystopian Ending

If all remaining empires in the world are Dystopias, the game ends. This is thematically similar to Warhammer or Mad Max where the world goes on fighting and into chaos, with the eventual death of humanity.

2. Utopian Ending

If all remaining empires in the world are Utopias, the game ends. This ending can also be triggered by adopting a specific doctrine after the reformation of the Congress. If the ending is triggered in this way, all remaining Dystopian empires take a massive one-time era score penalty. Thematically similar to Star Trek or The Expanse, where the world bands together to face the extinction of humanity, and comes out on top. 

And all other game end conditions enabled during setting up the game. 

MISCELLANEOUS 

These are some additions/conditions (both gameplay and non-gameplay centric) which I couldn't group into any specific category.

1. If the pollution threshold is breached due to nuclear war, no nuclear weapons can be developed and all existing weapons are disbanded. 

2. Dystopian and Utopian cities should look different from an artistic perspective. 

Dystopian Cities should look more akin to Blade Runner, Cyberpunk or Akira.

whereas Utopian cities should look more like, well, Utopias. Greener, more peaceful, happier, etc.

3. The tone of the game (ambient sounds, music) should generally become more bleak. 

4. In general, reaching the pollution threshold should be easier to actually be able to trigger the era. 

That's all I have in mind for now, I'm sure the devs and others can add to it.