Boneyard (L.A.) - yeah I know it was in the original Fallout, but that was over a decade ago. I think it should be redone to take advantage of the new technology, like how GTAIV redid the same city as GTAIII & the original GTA.
New York
Chicago
A city in China (Hong Kong, Shanghai, Beijing)
I'm not well versed on the whole Fallout Universe, but from what I understand SF is controlled by Chinese descendants with little NCR influence. Boneyard is part of the NCR so it may be devoid of the anarchy we're used to and might be a somewhat civilized area in a post-apocalyptic sense.
I'm also not sure what the state of affairs are in Chicago or NYC. And I know China was also destroyed like the US. But at the same time I think the series should be kept in the US for the time being.