I would also like to add something.
After amassing tonnes of data & knowledge on the Soviet role in WW2, why on earth would they scrap all that and start something completely fresh??....
well they didn't scrap it- they made this game. I'm sure they've had the capacity to research something else since then, if they wanted to.
Not saying they wont continue with WW2 of course- and obviously it's a setting close to their hearts, but it doesn't follow that it's gonna be the only one they are prepared to represent in a game.
The research process was probably rewarding in itself, and there may have been other scenarios that appealed to them, that they decided to research to a similar degree to produce this next game.
The COD4 move to modern combat leaves WW2 a bit more open again it's true, but the vietnam war for example is screaming for a new game IMO.
I'd be more than happy with a vietnam game hopefully with a campaign and/or co-op online, as well as a full RO style multi.
I suspect if it was made with similar realstic approach as this game, ppl would be interested in modding it back to WW2 anyway.
Conversely, if it is to be WW2- i hope this time round a team is formed up with the intention of make a Vietnam mod.