I think it's time for a break from the norm.
An East Asian game would be my pick - visually stunning and a cultural 180 from the current series. (A bigger concentration on hand-to-hand combat would be cliche, but fun)
Soviet Bloc /Eastern Europe setting would be a fun reveal. So far we haven't heard much about that part of the world in the cannon. Did they stay out of the war and just take over Eurasia when the dust settled?
An African setting would be very interesting - how did the war effect the Third World? Is Apartheid still looming its ugly head? (if so, confronting racism would be a dynamic storyline)
I enjoyed the thought of Mexico in NV. Along the same lines as Africa - how was Central/south America influenced? (The easy-out is cartels. Think more critically keeping in mind the alternate timeline of the Fallout Universe - cartels didn't really take root until the 60's and 70's).

I just had a thought - what about Pipboy and Vault tech?
Considering the Reds always copied the US and vice versa (and still do!), I think a PocketComrade doesn't sound too far out.

http://www.gamesas.com/index.php?/topic/1239668-fallout-4-speculation-suggestions-and-ideas-40/

There is a topic about possible http://www.gamesas.com/index.php?/topic/1216183-official-future-fallout-locations-suggestions-topic-8/, please use it.