I think that a organized group of raiders or bandits sounds feasible. Maybe a large group of cannibalistic tribals. Not on par with the Master or the Enclave but still dangerous enough to pose a threat.
I find it strange that there are no tribals in F3,but this has been discussed already so I won't.
I can see Raiders being much more organized though than they have been depicted. Surely they could have scavenged a large stockpile of Pre-War tech and the like. I could see their motivation being as simple as "survival of the fittest" and them not giving a shit about human life. Cannibalism was touched upon a bit in the series, so it's not a new concept, but cannibalism just seems practical if you think about it.
I remember the ending to "A Boy and His Dog."
Surprised the hell out of me, but it really made you think. How far would people go to survive. My guess would be "all the way."
Maybe it's just me, but if it came down to me starving to death because I can't find any food or grow any, you would be left with little choice, but to find some sort of creature to hunt or hunt your own kind. I guess killing "Evil" people would make it more appealing.
Anyway....
Perhaps a large coalition of Chinese troops. Surely there would be another group of Power Armor wearing guys out in the wasteland just aching to kill people and take over. Maybe they would hold some sort of resentment to the US, or maybe they just feel like discriminating against people different than them.
Racism has been touched on in Fallout with the Ghouls and Mutants, but perhaps it could take a more prevalent role with the Chinese. Maybe even a remnant of the Mexican or Canadian forces. I don't know what their tech level would be, but surely after the bombs fell people would want to search for other countries and their survivors to see whats left.
Since the Commonwealth was building androids maybe a android threat might be plausible. Especially with that kind of tech. I could understand a hate towards humans from an androids perspective since humans destroyed the planet.
If you want to go with Tactics as canon (like Bethesda eluded too in F3) then a organized Deathclaw rising might be a threat. Maybe you could even say that the Enclave was building up a force of Deathclaws to unleash on the Wasteland (perhaps with slave collars of some sort) and target specific races like Ghouls and Mutants. I know the Enclave has been beaten to death ,but they don't even have to be in the game. Just add in the back story that the Deathclaws were being tamed and used as slave warriors.
Not the best, but it's all I got for now.
