On the other hand, the further we get from blasted cityscapes and hard, hostile wastes, the closer the franchise gets to morphing from Fallout into an Atompunk Red Dead Redemption.
I think this is the real elephant in the room when it comes to future
Fallout games; assuming these games are set after the events of NV, the "post-apocalyptic" tone of the series is becoming harder and harder to maintain in a plausible (read: internally consistent rather than "realistic") manner.
The existence of the NCR shows that there are clearly enough people with enough resources now to begin rebuilding something resembling the old society, complete with a standing army and republican institutions; realistically I would expect that the age of scavenging from the old world is probably about over, and that people are probably now focusing on building things anew. I think this is why groups like the BOS, which are obsessed with hoarding old world tech, have become pretty much irrelevant; two centuries after the bombs fell, we see that there are still some sweet spots ripe for plunder like Repconn, Sierra Madre, etc, but for it to continue much longer seems unbelievable. There are only so many abandoned military bases out there. If the NCR and Legion can manufacture enough materials to equip armies, then it probably makes more sense for them to focus on developing their own forms of tech and culture, rather than relying on the diminishing returns of ruins and memories.
To me the biggest design/story-telling question that any Fallout title set after Vegas is going to have to answer is this: is it going to maintain an aesthetic of "right after the bombs fell" in the manner of FO3 just for the sake of retaining the "post-apocalyptic adventure" label, or is it going to seriously try and grapple with what happens once societies move beyond the immediate aftermath of devastation, pick up the pieces and build something new?
The only real ways I can see of dodging this question would be to set a new Fallout game earlier in the timeline, or else in an isolated part of the country with little to no contact with the West Coast; maybe even have it so that in this section of the country, the Vaults don't open until significantly later, so that we return to a setting more like FO1, with communities that are less developed and infrastructure that hasn't been touched yet.