Texas and Mexico is filled with all sorts of crazy factions, including a Texan Brotherhood, a bunch of AI factions in Mexico (the digital ghost of Santa Anna makes an appearance) and loads of others besides. The midwest, filled with a variety of different tribes and mini-nations Oregon and Washington (chock full of raiders and mutants. West Coast (NCR, Brotherhood, Caesar's legion, the usual suspects basically)
These guys have made something incredible. But after their recent update I've given the mod a try and wow. After putting it off for months, I'd written it off as just a re-skin. Recently I've tried the HoI4 mod Old World Blues, a WIP mod that's been recreating Fallout's universe in the HoI4 engine.