We need Terrorbirds; have mutated 'Chickens' or 'Fowls' that evoke their old glory and their cousins, these fuckers:
We need another rabid human mutant mob. Ghouls are running their course. Tunnelers were made too OP. Trogs were fun enough but can't be everywhere, they feel like the natural product of neo-industrialization and slavery. Lurks don't count (they're...crabs?)
Let's say there was a vault, or psuedo-vault. A religious one (invoking megachurches and their wealth) gleaming the fuck out of flyover country bumfuck county. They save thousands, nay, tens of thousands. They live for decades, or even centuries, down there. But they get twisted, perverted, maybe there is a revolution. They turn to their version of 'evil', maybe intentionally so. Maybe a team of attached scientists were used to make a 'common enemy' for the dwindling oligarchical/saved/worthy core to fear/fight off.
I'm proposing legit 'devils' and 'demons', and yes I'm shoving this in due to my love of such shit.
'Devils' would be the more intelligent, humanoid ones; while 'Demons' would be all sorts of warped, creepy animalistic shit. They both reproduce a lot and were thrown out of the 'Ark' or w/e into caves and the like and the scientists also wanted to 'adapt' to the new world so they filled it with a rough neobiosystem and maybe changed themselves to become 'arch devils' (e.g bosses), and maybe even were exiled or left of their own accord.
Devils and Demons can fight one another, Devils can amass and control Demons, Demons just run around in packs of their own hierarchy, whatever. They spawn a lot, they die a lot, they expand a lot, but they're never a world-ending threat, unless the Arch-Devils convene and unite and even then they're just...bioengineered/genetically engineered beasts and humanoids. They don't want to take over the world, they can barely concieve of it and probably just adhere to what the Religion
thinks they should do: rampage, seduce, enthrall, waylay.
They have have metal weapons or whatever they can strap onto a demon but nothing DOOM level with rockets or w/e. They might have chitin or horn or claws but otherwise against even a militia with guns or a cavaran they're not too much of a problem, if you know they're out there and keep a watch.
There's 'Hell', ground Zero or Ground Zero.1 where they were first born or took root but by now there's a odd collection of caves, caverns, towns, outposts they have changed, maybe three or so one per archdevil but nothing too big, but still a eyesore. The biggest megachurch in the US is in Texas but honestly it could be any interior-ish state?
How about mutated Pigs? where are all the Pigs, anyway? Pigs and chickens are mysteriously absent in Fallout, innit? We got the Cows, we got the Sheep now, so add Pigs. Either as Pigmen, who should move to replace SM as the 'Orc' type enemy (yey pig orcs) or just packs of boars being, well, boars, the fuckers. Drops good meat either way.
Other American fuckery for consideration for mutation: Wolverines, Groundhogs, Bison, Gila Monsters. Wolverines sort of fill a 'Deathclaw' esque slot, Groundhogs could turn into toothy pack monsters, Bisons a sort of more aggressive BigHorner type thing, Gilas might be filled by geckos and Dclaws already.