I would like to have seen Denver, Boulder, and Colorado Springs (and Cheyenne Mountain) being done by Obsidian if the old team had still been there.
I would not have minded the return of some of the discarded ideas of Fallout Van Buren but I would also liked to have seen new content. (Boulder would have to be something else now that Old World Blues had used the Pre War scientific center with space age technology, and Dead Money the idea of a toxic cloud. The Crater could perhaps have used elements from The Reservation, this Ghoul sanctuary where Ghoul scientists are working on ways for their kind to procreate as well as the means to stick it to the normies)
Rather than the desert wasteland of New Vegas nature has greatly recuperated in the Colorado region (due to the presence of the Nursery, the nature Vault done right), and it would have drawn in a lot of people other than the native tribes that are already here.
Denver itself would perhaps be a NCR boom town as well as a prison camp where prisoners can work of their sentence in return for prospecting the ruins. Still there would be plenty enough for regular prospectors as well, bringing in people from the West such as the player.
It would also perhaps be a contested area between the NCR, the remnants of the Legion and what other powers are found in this region. (I am not suggesting that it should be like this but perhaps the Midwest BOS could make an appearance though I am personally in favor of phasing out the BOS for new tech factions)
Detroit which
@TorontRayne has brought up could also be a good location for a Fallout campaign though I think it would be more interesting if it was set earlier in Fallout history, around the time of Fallout 1, in order to show how this region of Michigan would start out. (there would be no Super Mutants, no BOS, definitely no Enclave, just settlements one or two generations old, perhaps one fully functional Vault that just opened)
In my take Detroit before the War but around the time of the resource wars experienced the second economic miracle or boom when "manufacturing came home to America" after the US decided due to the resource crisis not to export manufacturing jobs to foreign countries (outside the US, Mexico, and later Canada) to prevent sabotage or theft of technology and resources.
Some of the first nuclear powered cars were manufactured in the Detroit car factories.
Detroit's industry would also play an important role in the manufacturing of weapons and munitions including the famed power armor. (the manufacturer had at least one R&D center in the region to work on improving power armor and design variants)
Lastly, it played an important role in the invasion and annexation of Canada as it was one of the locations along the US-Canadian border from the the Canadian campaign was launched. (so lots of military presence before the War)
I also thought that El Paso as had been made up by the Fallout Lonestar team sounded very interesting. They did some very interesting ideas with the location and the Fallout lore, suggesting that the Mexican Cartels would switch from drugs to oil smuggling before the War, and adding locations such as the Van Buren automated prison facility (a nod to Tibbet's facility).
Fort Bragg sounded like it had been re imagined to take the role of the Reservation from the cancelled Van Buren.
But like so many Fallout fan projects that one seems to have died.