Since RO:Ost was released, there have been a million theories as to what the next TWI game will be.
Regardless of what the subject matter is, there is one thing to consider: the business end of things. What this means is that whatever they make needs to appeal to a certain amount of people so they can sell a good amount of units. I don't mean they need to sell out by adding crosshairs and arcadey compromises. I just mean that the setting itself needs to be chosen carefully in order to appeal to the market.
It would be amazing to have a Winter War game, or a Romanian Front game, or a Japanese-Russo war, or an Afghan invasion, or a Cuban Revolution, or a Spanish Civil War. However, the fact is there are an unfortunate amount of people outside of this forum who don't know or care where Finland is (and that - wow- they kicked Russia's ass?), or who the Mujahadeen were. Zero awareness = zero interest = zero sales.
At least RO:Ost combined two countries that people were familiar with, Russia and Germans. They may have not known they fought each other (lol, RO as alternate history) but they knew who they were.
Maybe I'm selling the average Joe short, but in conversations over the years I've gotten the impression that apparently the only wars fought in the 20th century involved Americans.
The things I think are possibilties are:
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The Korean War: It has never been done to my knowledge, at least as an FPS. This is the last stand of "classical" warfare before missiles and satellites took warfare out of visual range. Lots of WWII weapons mixing it up with early Cold War weaponry. (Scoped PTRS= woohoo!) It has the Americans, so that right there is a market selling point. Also, it has many different types of battlefields, from mountains to marine amphibious invasions.
* Eastern Front Redux: A bigger, badder Ostfront. Stock usable mortars and AT guns, more complex destructible environments, a King Tiger for all of the whiners, improved physics, visible damage on vehicles, etc. Also, it could feature the addition of the other countries involved in the conflict, such as Romania and Poland. Plus, a campaign mode tied in with the streaming map technology that would allow servers to set the game up as a real war, being waged back and forth across the continent from battlefield to battlefield. I hope the latter gets implemented in whatever they decide to do.
* World War I: Bolt-actions, early machine guns, early tanks, early aviation, and some nasty trench warfare. It would feature major players such as the British, French, Germans, Russians, and Americans in one game. Hell, we'd still be using the Mosin Nagant on the Russian side.
I don't know how successful this would be since WWI is perceived as pretty stagnant (trenches) compared to the highly mobile combat of later wars.