[Kairu]

Perhaps we could say that New Vegas happened a year before FO3, and the NCR triumphed over Caesar's Legion. The Legion was pushed back to the rest of the Utah/Colorado (I think that's where they originated) and the NCR continued to expand continuing it's competition with the Brotherhood of Steel. In turn, due to common enemies, they have a makeshift alliance (or so to be) with the Enclave. And later on finding out about the Enclave's darker methods, separates itself from the Enclave causing a war of three major factions? Just a starting idea :P