Every game needs some kind of story and theme, even if it's just a space marine in Hell or a collection of action heroes in a battle royale for no apparent reason.
I have one idea (mentioned in the animation thread) for the setting: Alternate present/future, with a continuing Cold War with west vs east, Reagan era heroes vs Ivan Drago in the space age of 2010's. Proxy battles in Asia, deniable ops, space hotels etc. with plenty of room for robberies and such aswell.
This would allow us to have both classic Cold War 80's and 90's weapons (which everyone keeps calling stylish), present tech for the tacticool and future tech maps that shows off the engine in the same bag. Ofcourse, the Cold War is the next big thing that's gonna be over-exposed (it got a headstart with BF:Vietnam but didn't catch on until COD:Blops and now we have Magicka: Vietnam...), just like we've had the WW2 trend, then Modern Warfare and now recently near-future Warfare. So we'll have to choose how to approach that, maybe it's jumping the bandwagon but it's just one idea.