Well firstly I'd like to think that the Capital Wasteland in 2161 is still pretty fucked up, but not to the barren no man's land level we see it in 3. In fact, I'd have the outer areas of the Capital Wasteland appear much more similar to this flora overhaul for Fallout 3 where it's in this strange perma-autumn like state. As you get closer to DC, however, it turns more barren. Wildlife here would include mammals like Molerats (Naked and classic FO giant versions), Brahmin, Pigrats, Yao Gaui and a strange monstrous combination of what appears to be large feral dogs and possums. Insect wise you'd get swarms of Bloatflies, Bloodbugs and giant Ants with their giant termite like mounds. Other creatures would include Mirelurks and gigantic toxic Radtoads. Oh yeah, Wanamingos are back. Big time. As are their horrible critter-cousins. Guess what, they're from DC!
DC itself is much in the same style as the LA Boneyard or Bakersfield in that it's a charred art-deco nightmare, a broken visage of a "World of Tomorrow". There would be numerous "Dead Zones" throughout the city where all but the most terrible of mutants must avoid due to radioactivity. In the "safer" areas of the city you've got competing Salvager Crews that have set up their respective camps whilst they vulture the city for raw materials, bits of technology. Whatever they can find. At one point they used an honor system of claim staking, but in recent times they have brushed up against eachother to the point of violence and a minor-scale arms race due to various feuds, arguments and misunderstandings. Effectively think of a more blue-collar working community version of the Regulators and the Blades. In the midst, you also get "Claim Jumpers" (read: raiders) who will wait for salvagers to do all the hard work, then cap them in the dome and take the prime results. The DC ruins are littered with the signs of pre-war calamity. Heavily armed National Guard outposts. Anti-Riot "Instapens", quarantine zone maps. APCs rotting in the streets. Pre-War DC was a police state under siege.
DC Metro: I also like to think that in the Fallout world DC took the Moscow route and at some point in the Resource Wars undertook a gigantic effort to completely rejig the DC Metro. Digging much deeper, and wider. The problem was that when the bombs came, those who hid in the metros quickly succumbed to starvation and thirst. The surface was poison, and the metro had no resources stockpiled. Except for those in Vault 100, connected deep within the metro. After 25 years of use, Vault 100 began to suffer serious structural issues. A result of nearby construction of the "Deep Metro" that was kept (perhaps unneccessarily) top-secret from Vault-Tec under their unusually tight Government supervision. Vault 100 evacuated some of its population into the metro tunnels, taking whatever vault technology they could with them. Many, if not most, of the Vault lies inoperable. However, several floors remain in use. The "Dwellers" or "Metrofolk" have spread throughout the tunnels in the decades since, and with a growing population formed their own sub-communities. However, the growing population has meant that ration distribution from the vault's remaining hydroponics tech has become increasingly restrictive and controlled. Already there is strain between a newly forming social divide between the "Vaulters" and the "Tunnelers". After all, if a station on the far-end of the metro grows its own mushrooms why should it have to abide by the regulations and mandatory contributions of an Overseer that provides basically nothing for them? Even gangs of ruffians exploiting others in this time of rationing are starting to form, like the Tunnel Snakes. With outbreaks of the New Plague occuring and hamhanded quarantine procedures by Tunnel Security, stress levels in the DC Metro are rising.
Dwellers suffer the same issue as the Slags from Fallout 2 wherein they are defined by pale skin, an aversion to direct sunlight and a distinct discomfort at the open sky. However, conversely they're able to see in the dark far better than anyone from the surface.
Amongst the Dwellers, there's ghost stories and unspoken rumours of the "Deep Metro". The idea that there's a metro even larger than the one in which they live, even deeper below the Wasteland. Occasionally, if you're in the wrong place at the wrong time, you'll slip between the cracks and arrive there. A place inhabited by ghosts of the Old World, and once you're there, you'll join them. A handful of older tunnel scavvers will tell you a tale claiming they've been there and lived, if you give them enough booze at a station bar. More on this later.
Rivet City: The Capital Wasteland's millitary hub. Descended from US Navy crew from a battleship that was off-coast during the Great War. Little is known about the details of their "Grand Voyage", only that they were once far away, travelled to many places, and ended up stranded on the coastline of DC. The battleship they've built their community around, to which they refer to as "Mama Rivet" or "Big Mother" (In a fashion devolving into further tribal reverence each year) is in a state of active decay and ruin, largely inoperable and destined to split and sink into the sea in the coming years. Its people are heavily armed, utilizing Old World millitary weapons and armor, including Power Armor. It's a hard life in Rivet City, if you aren't willing to be a life-long soldier (or are too weak to make the cut), you are casted out or rendered an "outsider" like the other non-citizens that trade in the City. Their society is structured very much like that of a millitary unit. They subsist on Mirelurk fishing and Mama Rivet's generator provides ample power, but growing population and swathes of the New Plague mean they've had to look for help elsewhere. They tend to deal with the New Plague with a bullet to the brain, which works fine for civilian outsiders, but when one of their own boys gets sick. Well, that's a problem.
Megaton: A town built around an undetonated bomb from the Great War. Inhabited by a religious cult of Ghouls who believe their Ghoulish state is some sort of gift from an atomic deity, immortality. They revere the bomb as an idol of their Gods and their great gift. They welcome in Ghouls from across the Wastes who have experienced discrimination as a result of their misshapen form, assuring them that they aren't malformed, they're beautiful. Megaton is undergoing a religious schism between those who believe that Megaton and the Church of Atom should be a sanctuary for Ghouls to live equally, but seperately, from humanity in peace and those that believe they are superior to humanity, and that the bomb is not an idol, but a call to action. That they must detonate the bomb and irradiate the Capital Wasteland to make it "their land", unfit for humanity except for those who might transform in the aftermath of its detonation. There is also a Ghoul scientist, considered a madman by those in Megaton, exiled recently from the city for inhumane experimentation, attempting to find a way to allow Ghouls to reproduce. He believes when he finds the solution, he will be granted re-entry and hailed a hero.
Arefu: A town built on and along an Old World bridge, suspended above a canyon pass. The canyon below is a huge, termite-mound like nest full of Giant Ants. The townsfolk of Arefu have a system of pulleys and suspension ropes to send people down to crack open the nests of the Ants and retrieve their large eggs, one of which is enough to feed a couple people for a week. Occasionally they'll have to battle the Giant Ants, but usually the people on the bridge above yank the egg-miners back up before that's a problem. One kooky lady in Arefu has an idea for controlling the Ants with technology to make them not just docile, but obey orders.
Kingdom of Dave: Dave, and his father before him, and his father before him, is kind of a big deal. Overlord Dave controls one of the Capital Wasteland's main sources of clean water, from a series of water towers and resevoir, same as the Hub back in California. The Kingdom is protected by the Talon Company, a band of mercenaries from up north, paid for in water and caps. Overlord Dave rules his domain (Which includes smaller settlements like Arefu and Cantebury Commons) with an egotistical and petty fist. Or did, until he came down with what everyone believes to be a case of the New Plague. He now lies sick (although a smart player might deduce that he's been sick longer than it takes New Plague to kill someone), his fate uncertain, as chaos rules his court and subterfuge occurs between his potential heirs. Drug use is rampant within Dave's Kingdom, the Overlord himself being a regular Buffout user.
Oasis: In the remote parts of the Capital Wasteland, a hidden community lies. A primitive, agricultural community. Wooden tribal houses, tents, campfires burning. The entire place entwined by a strange, vibrant vegetation. The outskirts of the village are protected by ancient patrolling robots. The plants here produce large, succulent fruit that the village subsist on. The centre of their village is a large, tree-like mound of twisted roots and ivy - forbidden to anyone but the village elders. Beneath the tangled vines lies an Old World facility where a pre-war company was attempting to solve the food shortages in the country with a Soylent Green wannabe plan: conversion of corpses into vegetation. It turns out the village continues this tradition: feeding the corpses of their dead into the contorted green fleshy mass at the centre of the roots. Unfortunately, it seems as if they're getting less yield of fruit for their corpses as time is passing. To them, the tree is demanding greater sacrifice.
Tenpenny Tower: The Capital Wasteland's den of sin, and also its home of luxury. The esteemed Mr. Tenpenny began as a mere Raider chem cook in his youth, but the success of his enterprise overwhelmed him. What was once an Old World hotel is now a den of gambling, whoring, drugs and boozing. A kind of "No Man's Land" for the militias and raider gangs of the area, all of them reliant on the massive chem production going on in the lower floors of the tower. After having watched more than a few Old World tapes about foreign aristocracy, Mr. Tenpenny (Formerly known as Crusher in his youth) adopts a new accent and runs extravagant parties in the upper floors with his selection of socalites, slave-harem and the whose-who of the Wasteland (Including Overlord Dave and the Commander of the Talon Company). It's a non-stop party up above, fueled by the chaos and depravity down below.
Paradise Falls: Not much to be said about them, truthfully. A band of slavers sent from the far-away Pitt to gather slaves to send back home. They also sell to Tenpenny and Kallos.
Cantenbury Commons/Little Lamplight: Cantebury Commons is a community guided by religion. A pre-war religion by the name of Hubology. Although they've got their own twist on it after a few decades of fragmentation from the Church. Children aren't treated well in the Commons, which during the chaos of a Wanamingo attack, led to a long-planned exodus of the town's children to escape their horrible parents under the leadership of MacCready. Heading to a distant pre-war tourist trap, "Little Lamplight" (discovered by MacReady in a pre-war magazine), the children have been trying to eke out an existence. Poorly. Things are going the way of Lord of the Flies, and soon it won't be long until disaster hits. The player could help shape them up into a real chance of survival, report them to The Commons, report them to Paradise Falls, or incite further juvenile chaos.
Kallos: (AKA cannibalizing Van Buren ) Prior to the Great War, Med-Tek (Somewhere between a subsidiary and sister company of West-Tek) had established operations in the DC area with one of its patent "pop-up" facilities that had spread across America like boils in response to the New Plague. These state-of-the-art space age looking facilities would act as research and response centres, and the DC facility was one of the most developed next to the Boulder, CO facility. The nucleus of this science-whizz cell was a ZAX computer unit known as "The Director" which organized and structured response operations with impeccable precision and speed. Due to its "Pop-Up" nature, it wasn't a target of the bombs. When the news came that the bombs were coming, the scientists of Med-Tek came up with a desperate plan: Entomb themselves within the gel-stasis floatation tubes that had previously been used to artificially prolong/freeze New Plague patients in order to study them, entrusting The Director to discover a way of removing them from gel-stasis alive and cranial functions intact. To decide who would die, and who would take a space in these tubes, they referred to the Director to organize who was the best qualified for survival. They did so, and so the bombs did drop.
Twenty years later, in the rural mountaind, the scientists awoke to the Wasteland. In the years since, the scientists formed a budding society with the Director as its sovereign leader. Using the technology of the facility to create protein farms (ala Blade Runner 2049), its robots as defensive measures. The outskirts of the facility occupied by Wastelanders. Eventually, the old problem of the New Plague reared its head once again in the Wasteland. Using their pre-war tech and a Director who spent decades thinking of nothing else, the scientists of Kallos were able to formulate anti-dote treatment, but not a vaccine. Production of this anti-dote is a complicated process, and with the passing of the Scientists, their child-heirs have relied on the Director from which they claim their authority above the townsfolk outside. The problem is that the ZAX unit has begun to cannibalize its own memory banks, and did so from around the time the original scientists woke up. They had kept it prolonged with scavenged technology bought from the DC salvagers, but it has recently shut-down for good. The "Scientists" in the dome are now left to run a Wizard of Oz show, formulating the (limited amounts) of antidote out of a ritual-like memory rather than true understanding, and pretending that the Director is still alive. To make matters worse, the material required for the antidote is dwindling. Kallos has recently entered a pact with the soldiers of Rivet City: Antidote in exchange for electricity and protection.
Raven Rock: A Pre-War Millitary Installation used to develop cutting edge weapons technology, namely cybernetics and bio-organic weapons (Deathclaws and Wanamingos). The facility was hit hard during the Great War, and rendered into a blackhole of radiation much like The Glow. However, in the depths of its guts, the experimental Wanamingos (Co-Developed at a facility outside of Sacramento, CA) thrived in radioactivity, becoming malformed, grotesque mutant versions of themselves. The place now is a bio-organic goo hive of Wanamingos, where swarms of the creatures emerge and set out into the Wasteland. The latent radioactivity causes wild mutations, resulting in numerous "types" of Wanamingos vastly differing in physiology. Its the imperative of all Capital Wastelanders to wipe out any Wanamingo gatherings they come across, because if they settle for too long, they'll begin to form another goo nest. There are horror stories of the Wanamingos carrying off Wastelanders back to their nests, to make them "part" of the goo.
"The Deep Metro" AKA Congressional Vault:
The Dwellers are right, there is a Deep Metro. Before the Great War, the US Government in its expansion to the DC Metro system and construction of Vault 100, further commissioned the construction of an additional, deeper (but much smaller) metro network intended for US Government VIPs. This was under the supervision of what would come to be known as "The Enclave", at the time an unspoken group without a name. The plan was that the US Government, including the Senate and the House, would retreat for safety underneath here in a Vault-Tec blacksite. As the situation...changed.... nationally and internationally over the years, the plan was altered when it was assured to "certain members" of Congress that the relocation of the President to the Poseidon Oil Rig in the Pacific was to ensure better survival of the US command structure: to stop the ChiComs from cutting the head off of the snake in one move. Truthfully, it was actually a purging operation. Those who were truly important (not just the politicians) to America, those who would be absolutely loyal and singular of mind, no deliberation or debate, were the ones who would retreat to the Oil Rig. There was no need for Republicans or Democrats, only the true masters of America. The "True" Enclave. The "rabbling mass" of Congress, the pencil pushers and those who "thought" they would be important, would be the ones put in the Congressional Vault originally intended for all of the US government hierarchy.
The Vault lies empty, now. Its residents all dead. The facility itself, still pristine and full of technology, weapons, Power Armor. It seems the divisions of Congress dredged up to the surface in the face of the apocalypse, when in that great silence all they had to reflect on was the sins of their government and their decisions. Even when the world ended, their hatred for eachother bubbled up. It appears there was some internal unrest, conflict, and some sort of coup or maybe a trial. Hard to tell. Here the player would find the most comprehensive and clear log of the actions of the Pre-War US government, and the logs of feuding, betrayal, petty conflict until the Vault lay empty. The tunnels of the Deep Metro patrolled by robot guards protecting nothing. Think of a combination between Vault 11, Mariposa and The Glow in terms of tone.
Miscellaneous:
There'd be smaller communities dotted around too. Your odd fishing village or farmstead, but these would be the key ones. Same with there being a smorgasbord of weird raider-gangs out in the wild. I always liked the idea of a tribe or faction that was entirely nomadic and lived only by the roads, leaving stashes and messages for eachother. Like Wasteland gypsies with a Road Warrior aesthetic.
You'd still have some of the Vaults like 112 (I like the idea of the Stepford Wives VR reality nightmare, but I'd go a step further and have them be brains in jars wired up rather than preserved human bodies.) and the player would still be from Vault 101 which would still be a dystopian 1984 rip-off, but instead they'd probably just be a vault dweller exiled for a crime that the player can choose if they did or did not commit.
The main quest:
I have no clue what the main story would be. Something to do with Wanamingos and New Plague.
Coming up with "Main stories" has always been my weakspot. All my campaigns I've ran have been sandboxes or faction conflicts for this reason. I think maybe you could do a factional conflict over discovering the cure for the New Plague (i.e who controls distribution) or you could further mutilate and recycle Van Buren's plot line with ODYSSEUS and the BOMB-001 platform, with the "big reveal" being that the spread of the New Plague wasn't a naturally occuring pandemic but rather something planned by a sinister figure, or you could combine both and have the factional powerplay as an undercurrent.
You could do something with the Wanamingos too since I sort of wrote them as an existential threat to the Wasteland, although I think that'd be quite boring as a premise.
Or you could even recycle the main plot from the real Fallout 3 in some way, like having the player exiled from Vault 101 and picking up the trail of other exiles who are working to restore clean water to the Wasteland. Or you could do what the real Fallout 3 did and recycle previous plot points, but good ones that lead to cool storyline branches like the player being sent out to retrieve a vital component to save their Vault and running into the bigger storyline. Shit, I don't know.