I'd like to see a WW2 game attempt it though. You'd use an episodic model. The base game has like 2 campaigns, probably American and Soviet or something, for about 6 hours of content which is standard for a shooter. Plus multi modes etc. Then every 4-8 months you release a new campaign, 2-3 hours long focusing on a new part of the war.
In the base game you'd have D-Day and Stalingrad covered since everyone wants them, but then later on you get to explore more unknown parts of the conflict. Chinese campaign in the Second Sino-Japanse War. Finland in the Winter War. Partisan campaign for Yugoslavia. French campaign in 1940 prior to the fall. Even outright axis stuff too - German campaign covering Poland, Norway, Low Countries and France. Romanian or Hungarian campaigns.
There's a lot of potential to be mined for interesting game settings, but the main problem is that it's not economically viable to theme your game around something obscure. You usually end up catering to American audiences, throwing in bones for the British and Russians, and after that you've run out of money to make your game. You can only squeeze so much content into a conventional game model, even if you have an expansion pack or whatever.