Well, I was not the first to put the idea forward, I just made the link between your ideas and @Orlanth ones. As I’ve found back the link, he spoke about it here then later here. Hopefully he may intervene here also now.
Not a lot more to add frankly, (edit - ok but then I started typing) but some clarity on my emphasis could be helpful.
Adding an endgame challenge is only half the benefit. It is directly historical in relation to a city game based in the Bronze age, or continuing through to same, because such was the fate of the cultures in the Levant and surrounding area. We also know some cities survived, even isolated ones. So the Bronze Age collapse was not universal and thus by skill the player could ensure their city weathers the collapse.
On a game design point we dont know how bad the Bronze age collapse was, so there is a lot of leeway for a slider, we do not know what it consisted of, but we do know that waves of invaders were a part of it, as was a drought (likely brought on by climatic changes caused by Thera). The devs have enormous leeway to implement the catastrophe directly and indirectly.
It is also cheap to code. Presumably the game will already include natural disasters and drought, if the game cant handle drought the game will suck. So I will assume they can, there is no excuse for non implementation at launch for Bronze age expansion because at a very minimum the game can process a long drought and simultaneously turn off the trade function - blaming Sea Peoples.
There is no need to render a single Sea People or tsunami and can still feature the Bronze age Collapse. Hopefully we can have more later.
Now for an end game disaster to overcome for the purely neolithic period we can have same. There is less evidence as we dont have an event to go by. The transition to th Bronze age was slow and not marked by any transitional event. Indo Europeans will have been arriving for a while, it will not be sudden. There is nothing stopping an end game event being scheduled for the players city though.
My suggestion here is that in the final time of the game a natural disaster is scheduled, triggering an incursion. Exactly what could be randomised. Natural disasters should happen during play anyway, and they should be unforeseen and random. Just have a big one at the end. he disaster need not even happen to the player city. You could for instance as an end game event have an earthquake the damages the city walls which is followed by an opportunistic attack (like Troy 6). Or another city could be leveled by an earthquake and the survivors decided to encroach.
What we know about larger disasters is that they compound, have lots of dead you get disease, have damaged cities from earthquakes you also get disease, or exploitation or loss of trade. The list goes on. If the devs have done their job well by release they will already have the tools to craft a late game challenge. The only difference between that and the Bronze Age collapse is that is is local and a-historical (but based on the realities of the age).
Now an end game event has two purposes. The first is the ‘boss’ challenge, which is obvious enough. Rack up the difficulty right at the end as a finale. for Bronze age games this should be frankly mandatory as surviving the Collapse is the event you are preparing for all the time of play, and marks whether the player has the chops to make a lasting Ancient City.
Every other setting should have the same, utilising whatever historical disasters we know of as flavour, but just ramping up the general misfortune otherwise. In addition to making a challenge it should also alter the city this second point is important.
Games of this nature tend to stagnate, the streets you build in early turns as the same as later turns just with upgraded buildings on them, having a truly evolving city should be encouraged, and that naturally involves decay. Look at the maps of medieval city and you can trace the modern streets, on most, because they endured. Troy however was rebuilt time and again, and the different levels of the city had little relation to the plan of former cities.
Transitional disasters clear or partly clear the map for new growth. In the best cases you should see parts of earlier iterations of the same city in the game map as it evolves. Only disasters properly allow this. It is a good habit for the game.
A word of caution here, when Sid Meier’s Civilization was first developed there were periods of stagnation included, but when players encountered them and were setback they restarted the game and very few games were continued to completion that way, so the idea was dropped. However we can learn from this, an end game event doesn’t have that dynamic,you build your society with the purpose of facing same as the player and the setbacks are easier to swallow. Secondly Sid Meier’s Civilization was a very much simplified god game, which didn’t have much interaction between game elements. Each unit and city was a piece on a game board, while Ancient Cities has the dynamics of a sim, and thus can develop personality.
Back to the end game catastrophe though… The second main reason to include one is to answer the players question of ‘what next’. Most games of this genre have option to play on post victory, and few do as there is nothing to really do. Lets take Sid Meier’s Civilization as an example, once you have conquered the world or got to Alpha Centauri there is nothing left to do. So when someone did play on to the year 3000 and found the world would naturally stagnate to a radioactive ecologically devastated wasteland it made the news.
Players play to the finish line then end.
So you need a finish line. But more than that by having the Collapse to weather, or a localised end game disaster complete with external and internal devastation you naturally have post endgame play option. In most games a winning position naturally surrounds having a maxed out game position. All critical buildings maintained, positive economy, tech tree complete etc etc. However a Collapse scenario doesnt result in that, in a Collapse you lose many buildings, your population degrade, skills are lost etc. Winning the game involves exheeding a survival timer to the finish line as the situation gets worse and worse. It will be hectic, it will be novel, it will be unique in games of this genre.
Instead of having multiple barracks and granaries, you get to choose whether to save either your last barracks or your last granary as much of your progress turns to ash around you as you try and make do and outlast the catastrophe.
Unlike Sid Meier’s Civilization where a setback meant a player reset, the playerbase knows that all they need do is hold on for a number or turns/game time/generations to win.
But there is more, when the finishing line is crossed the very nature of the endgame opens up additional play without any additional content required, because post victory a player can choose to rebuild those granaries, temples and barracks. Restore their city and they can choose of themselves when to actually say they are done playing and end the game.
Furthermore this wil be different in the Collapse scenario. With localised disaster endgame play can return to normal post endgame. For the Collapse things will be different, the Sea Peoples may be gone, but the trade network is gone too. You can rebuild, but you will have to scavenge materials you would have imported pre-Collapse. As a game feature scavaging will work as a city with no tin with no mean to import tin can scavenge tin from ruined civilisations.
You could have a whole section on post Collapse gameplay naturally evolving from the Collapse scenario.
In a nutshell an end game disaster would not only act as a capstone to play and a climax to the story but will also open up different styles of play as and of itself. Longer time games with multiple calamity scenarios become possible and neatly transition the game and act as hurdles to progress far beyond looking at a pretty city and thinking you have done well, looking at a scared rebuilt and scarred again city will mean so much more.