How would you go about explaining where all these companies were in past games?
Even NV had to make up the rather ridiculous contrivance that Sunset Sarsaparilla had absolutely ZERO market share in the north-east, to explain why we didn't see it in Fallout 3, despite being a major soft drink manufacturer on par with something like Pepsi.
Cincinnati would be my pick.
-Its on the border of three states, Ohio, Kentucky, and Indiana.
-Its in-between 3 major cities, Indianapolis, Columbus, and Louisville.
-Two of those three cities lead to areas we know are inhabited by large powers, Columbus leads to The Pitt, ruled by Ashur and his slave army, and Indianapolis leads to Chicago, held by the Midwestern Brotherhood of Steel.
-It would be fiarly easy to make up a power in the south, based in Nashville, and have them come up through Louisville.
The main plot would be each of these three powers moving on Cincinnati due to some mcguffin resource/weapon, and that the resource/weapon from Cincinnati, when comes with resources from each faction's home city, and the city between them and Cincinnati that they control, would give them to power to beat the other two factions back, and control the whole area.
Ashur has already shown a desire for manufcturing, the reason he decided to rule The Pitt was becuase it had the only working steel mill the BoS had come across, so it makes sense he would want to try to expand across the Amercian Rust Belt, looking for more factories, and the MwBoS could try to use those same machines for a similar purpose.
It would be like NV, but if the plot of NV mattered becuase each faction had a future instead of being predesposed to self-destruction.