Maybe it's because I'm British, or maybe it's because I would like a change from the US (afterall, this affected the world pretty much), but Canada or the UK would be interesting. I think London would be a good setting, plenty of underground systems, a large river running through the city and decent landmarks (I said decent, not amazing!). Plus, it would give Bethesda to craft something a little different.
A nice idea I heard from a friend who is a fan was, because the UK didn't receive so much of a pounding from the nuclear weapons, it may actually be infested with more greenery as opposed to becoming a complete wasteland (although certain parts would theoretically still be). It would give the setting a bit more variety without destroying the IP's mood. I mean, constant wasteland is true to how it would be and all that in America, but gameplaywise it's a little boring.
I had the same gripe with Oblivion - it was almost all forests, only the cities had any real variety, the rest was just forest more or less and it got boring to treck across. Of course, Skyrim mixed it up a bit with some snow and plains as well as forests, it was just nicer to stroll across.