One thing I think should be in a game called Civilizations is stagnation, degradation and ultimately collapse or fragmentation. I have always found it unrealistic that civs only progress on technology and culture, populations will always grow as long as there food and housing. But history shows that is not the case. Look at the current birth rates in the western world.

Civ6 does try to address this with loyalty and happiness but its influence is not very meaningful. Barbarians are used to represent the ununited natives but they usually don’t present a real threat. Golden/normal/dark ages kind of does this but it more piggy backs off the loyalty system and it doesn’t really have that many downsides, just “not as good” benefits. Nothing that really threatens you.

I think a system that implements a generational cycle like the Stauss-Howe theory of: high, awakening, unraveling, crisis would be the easiest with each lasting around 20-30 turns.

Like during a high (overcoming a crisis) you have extra amenities, huge production, buffed religious spread, more faith generation, fast pop growth (ie the baby boom), but a penalty on culture and science (conformity reigns during a high).

During an awakening is when people are sick of conformity and start to express themselves. You have higher culture and science yields, less production bonus than a high, lose some religious pop, penalty on faith generation, normal population growth.

Next is unraveling where individuality is exceeding solidarity. Keep the benefits of an awaking but the penalties to faith and population are even worse. Whole cities can lose their faith. But outside religions are also harder to spread to you. Unraveling is just as it sounds and religion (of any type) is usually the first victim.

Crisis is a bit harder to implement but it’s when the shared culture is in such a bad shape that there is no shared value system. Anything can tip it to the point of collapse. Maybe negative amenities, huge growth penalties, military units randomly disbanding, insurgents popping up in neighborhoods, barb camps rising up closer to your cities, decrease in loyalty in all cities.

If they make it out of it they get to enjoy the survival of their nation/empire in a high. If not then they split in different civs like what happened in older Civ games (I miss that mechanic). And either learn to live their new neighbors or start a civil war to reunify.