I think we briefly talked about this before the start of season 2, but decided not to do it because it would mean people would need to know certain bits of history.
I noticed there is a sub doing something similar on Reddit, /r/ColdWarPowers. Rather than it being more difficult to do than the normal ModernPowers, I think it might actually eliminate a lot of issues we had moderating it. For example, there is no way anyone can dispute something we know to be historically accurate, whereas people had all kinds of interpretations over stuff happening right now, and it was even worse once we got 20-30 years into the future.
The basic idea is we would start at the very beginning of the 20th century and go from there. The main difference between this and the current version of MP would be that certain historical things would definitely happen, no matter what players did. Both world wars, and maybe stuff like the great depression would still happen without any chance to stop it from happening, but they could play out in different ways. If we were going to do this, we would need to write a different ruleset, with a list of things that would definitely happen in-game.
There were also less countries in this time period too, so we wouldn't need as many players. There are probably 30 countries that would be worth playing as at the beginning of the 20th century.
The premise is very interesting, and I definitely would be down for playing this.
I like your idea of having certain scripted events happen, but I wonder if we could make certain events happen if conditions are right_, and only make them extremely likely rather than required. This might allow players to arrange for major events - like the World Wars - to still occur, but perhaps the details and alignments are changed in some way.
So, take World War 1 - I think we can generally agree that the assassination of ArchDuke Ferdinand was the major catalyst for the war; the assassination led directly to the July Crisis, which prompted everyone and their mother to declare war on others. So, we can leave in that event as a "mandatory" historical event; the assassination prompts World War I if other conditions are appropriately met. A list that is, in my mind too short, but captures the idea would be something like this -
World War I occurs if any two of the following three conditions are met:
In this way, WW1 is still highly probable, but the shape and look of it may change. The assassination of the Archduke might be a major event during the war, if players are hostile enough. The alliance webs might also change - instead of the central and allied powers, we might have a different set of allies, or even three factions rather than two.
You can take this idea of conditions and apply it to almost every major event - make it highly probable in some cases, or maybe just lucky in others (we can mirror the ways history might have diverged in this way too), and then align the conditions appropriately. You can take it to the Great Depression, the rise of communism in Russia, along with the fall of the Tzars, Hitler's rise in Germany, the arrival of America and the Soviet Union as the only superpowers following WW2; you can make these events occur in a way so that the general idea is there.
Maybe we still have a cold war, but maybe Germany won WW2; having no good way to invade America, peace is eventually restored with Germany and Italy in control of most of Europe, Russia beaten back a long ways, and America having dominance of the Pacific after their victory over Japan. Now the cold war is between the European juggernaut of Nazi Europe and the United States; the race to develop ever more powerful nuclear weapons continues, and the proxy wars continue, but the ideological battle is between fascism and democracy, instead of communism and democracy.
I think making choices like this, so that the big, history-defining events still occur, but can take different forms, hits the right balance between a cohesive narrative that makes sense and player agency, where their actions and choices still matter.
A second idea I had was to introduce slight randomization of times on historical events. For instance, Archduke Ferdinand was assassinated on June 28th, 1914. But destabilization in that area had been going on for a few years before that, so it could have happened earlier; and perhaps things weren't so bad until a few years after 1914, so maybe the assassination occurs behind schedule.
For this idea, it's a bit more straightforward. Take a major historical event, and figure out a decent time period where it could have occurred. For the assassination, let's say 1908-09 (the years of the Bosnian Crisis, precipitated by Austria-Hungary), and maybe make it happen anywhere in a ten-year period. So from Jan 1909 to Dec 1918, the assassination can occur at any time during this period, and it's chosen randomly when we hit the period, and not revealed until it's time. This is a straight-forward randomization exercise; the day doesn't matter for our purposes, so we roll a die for the year and a die for the month, and we have our assassination date. So maybe WW1 occurs a bit earlier, or a bit later - but as a mandatory historical event, it always occurs; but no one can be sure when.
You can take this idea for every major event and give a slight twist on it - maybe the Great Depression doesn't really hit home until the mid-30's, possibly making WW2 an even more desperate affair (imagine the world still dealing with a depression as the war breaks out!). In this way, we can all brace ourselves for the bad shit we know will occur - but we can't pinpoint the month or year it will happen.