Fortunately or unfortunately, if you keep to the same idea of "Desert Rangers", then you're more or less limited to... well... the desert. Somewhere. Probably in America, though rangers wandering the Saharan wastes would be kind of cool. (Although African geography is probably not most peoples' strong point, so locations would not have much significance.)
I would like to see a Wasteland game set in the jungles of Central and/or South America. This obviously negates the idea of a "Desert" Ranger, but nothing to prevent another US Army unit from being stationed in that area and left to its own devices. To my knowledge, this setting is not largely explored in most post-apocalyptic fiction, and I think it has a lot of potential. Mutated jungle animals, native tribes to either befriend or defend against, strange abandoned temples and lost cities (allowing for a bit of a mystical connection, if you like), and even secret hidden underground military bases (those work anywhere). Would also allow for some different skills ("Jungle Survival", "Reptile Wrangler", "Improvised Boat Building" etc...) and weapon types (not a ton of energy weapons in the jungle, I'd imagine, but maybe some creative types of explosives or melee).
Timeline-wise, I liked the original game -- close enough to the blast to ensure desolation, but far enough to allow some large-ish communities to emerge (and some fringe encampments as well).
I honestly like the desert, but after Wasteland, Fallout, Fallout 2, and New Vegas... I'm a bit tired of it. Fallout 3 had a neat idea in principle (urban metropolis after the blast), but for some reason I just didn't find it as engaging as I had hoped. Maybe it seemed too artificial; there's something about the idea of mankind being returned to a pseudo-primitive state and having to survive against nature as well as the new threats.