I've been coming up with a bunch of ideas for a potential Fallout game set in the way future (~2340s) revolving around the growing tensions between two very strong empires, the Khan Empire and Imperium (the successor state to Caesar's Legion).
The Khan Empire would be a massive and decentralized nation, with land reaching from eastern Washington and Oregon to western Iowa, control Wyoming, Nebraska, southern Idaho, and southern and central South Dakota. While most of the empire is under the rule of a governor appointed by the Great Khan (emperor/empress), there would also be several autonomous regions that are basically separate countries. One I came up with is called New Utah (northern Utah and southeastern Idaho) which is made up of various New Canaanite, Dead Horse, and Sorrow communities that fled southern Utah during Legion campaigns that happened in the late 2280s and early 2290s. I think that the current Great Khan should be an elderly Melissa Lewis, going by the adopted name of Khutulun (to seem more culturally Mongolian).
After the Legion's defeat at Hoover Dam (by House) and the death of Caesar, Lanius takes over and begins campaigns into Utah as a way of trying to continue to keep the Legion's war machine alive. This is decently successful until Caesar Lanius is killed by a fire bomb in battle. There is no clear line of succession beyond Lanius, whose claim was already a little shakey. Lucius declares himself Caesar in Flagstaff but many disagree with the idea. Almost every high ranking member of the Legion believes they have a claim to being Caesar and most travel to their home regions to gather armies to try to push their claim (like Aurelius of Phoenix, who goes to Phoenix and Antony who goes to Denver). Caesar Lucius sends his armies down to Phoenix and orders Aurelius to stand down and, when he refuses, the army attacks. Aurelius's troops are defeated but the tension has been released and many parts of the Legion rise up in open revolt. Throughout the rest of the 2290s and the entire 2300s and 2310s this war rages on. Lucius maintained the largest army within the Legion but there are still sizable opposing armies. Vulpes and the Frumentarii maintained allegiance towards Lucius but have been operating independently, perpetuating the war and strategically getting rid of people close to Lucius to keep him reliant on them. One day Lucius discovers that Vulpes was behind the assassination of one of his closest allies and has the Frumentarii cast out, executing most that aren't undercover. Vulpes escapes to New Mexico where he finds who he considers to be the perfect heir to Caesar, a relatively unknown legionaries from New Mexico who is a capable leader and has some physical similarities to Caesar. With Vulpes's help, he brands himself as Caesar's son, renames himself Augustus, and begins to accumulate a following, with Vulpes strategically assassinating any political or military leader who could threaten him. In the mid-2320s, Augustus defeats Lucius and successfully takes over the Legion as a puppet for Vulpes. He sets about rebuilding the Legion, which had been absolutely torn apart by decades of war, declaring it an empire named Imperium (still based out of Flagstaff) instead of a wandering military. With a strong leader, the Legion soon stabilized and began to grow as a threat to the Khan Empire to its north.
The game would center around the conflict but would have notable differences from NV because of how either way you're supporting an authoritarian empire. I think the civil war is absolutely necessary for the Legion to give different areas more of their own regional identity, since the Khan Empire will have a lot of that.