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.
20th century history definitely interests me more than the present. One thing we would absolutely need to have is a good battle system though. The current one would just not be able to handle simulating both world wars.