I am personally convinced Bethesda has an absurd timeline/internal mindset with regards to Fallout, which is exacerbated by their relative apathy to it compared to something like Elder Scrolls, I think there's an argument to be made for what them shifting a bunch of concepts and stories around to make them fit some original framework they had in mind. It doesn't involve the West Coast or central US whatsoever, and there's some vague possibility raised by this on what the hell Fallout 76 was originally meant to be. Important context for all this is every idea they've had for the Fallout franchise was more or less envisioned between 2008-2011
Sarah alludes to Virginia in her dialogue at the end of Broken Steel, just in case you were wondering how early Virginia in Bethesda's version of Fallout was conceived (2009-2011).
So, what happens to the Brotherhood now?
If you think this means the Pride has nothing to do anymore, you're crazy. We've still got the damn Frankensteins crawling all over D.C. and Rothchild's got us picking up every blinking gizmo from here to Virginia. Quite a bit of work cut out for us, eh?
This does help develop one potential ass-pull of a theory: Fallout 76 was originally the sequel to Fallout 3, set 3 decades after the (probably) original year for Fallout near the end of development i.e.
Fallout 3 2246 -> Fallout 76 2276.
I've theorised that 4 was intended to be a prequel to 3 prior, and by shifting 76 into a sequel, you end up with something like this:
Fallout 4* -> Fallout 3** -> Fallout 76***
*Grandson of John Maxon arrives in Boston after the BoS raid on The Pitt, his son Arthur is born during or shortly after the events of this game. There would be some allusion to a BoS chapter in D.C. assisting on a secret project, or not.
** A decade later or so later, after the events of 4, a much more diminished BoS is confined to D.C. The grandson of John Maxon, Arthur, is 10 years old. The BoS has some nebulous internal stuff going on hence their diminished presence in Fallout 2. The Enclave at Raven Rock emerge as the Western Enclave are wiped out between 2241-2245 (or are Enclave who left East). Eden possesses an ad-hoc or refined version of the F.E.V virus developed on the West Coast, and is meant to be an A.I. copy of the previous Enclave president.
*** 3 decades after the events of Fallout 3 (actually 4, but actually 2.5, but actually F1.5, but actually F0.5) in Fallout 76, Vault 76 mysteriously opens, just as the BoS and Enclave mysteriously appear in the region, resuming their conflict from decades prior. MODUS represents the core Enclave schism from 3, of serving an artificial intelligence (Eden) or being lead by flesh and blood (Autumn) with the MODUS Enclave being composed of the former, making them the true antithesis and anathema of the BoS. Vault 76, like 13, was a Vault The Pitt is post-Ashur, post-cure, divided into 2 factions vying over the city - in a sequel context you can be vague over the player's choice since both sides wanted to distribute the cure/vaccine regardless, and similarly be it Werner or Ashur the power vacuum after would provoke a split regardless).
In order to course correct 76, you would need to retcon more or less the entire story of 4 into being a prequel, which isn't too absurd given it originally was.
According to Emil himself in 2018, "Fallout 4" was originally going to be set closer to the pre-war, exploring a world devastated by the bombs. I don't know what happened here but given the writing finagling I think occurred with Fallout 3 and Bethesda's notoriously long dev-time, their prequel spin-off plans might've been brought up purely to create a sequel rather than a prequel out of worries it might've displeasured fans. This, humorously enough, still puts the blame on Emil Pagliarulo. Emil, I think, has the same issue a lot of Hollywood execs/producers/directors/writers do when it comes to videogame stories, where doesn't conceive of them being good couched in their own material, so he creates a story independent of it, then forces the setting to match. The more say he has, the less coherent everything becomes.
76 being released with such a hollow story (if you can call launch 76's "main quest" a main quest at all) and gimmick multiplayer can be the consequence of its actual, original main story being ripped out.
So in sum: This culminates in my favourite, total ass-pull of a conclusion: Fallout 76 was the original Fallout 4, Fallout 4 was the original Fallout 76. The settings were switched as a consequence of the haphazard treatment of the franchise and fears over possible backlash to a prequel after so long a wait.
Fallout 4 was in development before Emil was made head writer if you didn't know, and I think he forced what was already developed and adapted it to something else, which this assigned responsibility more or less being a result of his work on Oblivion and Skyrim. A proto-Fallout 76 was turned into Fallout 4 haphazardly. Emil might not be totally blameless, since I think Bethesda decided it to be a sequel last-minute with no multiplayer component due to the disgruntlement toward ESO at the time. It might also explain the relative lack of Emil's trademarks, such as a heavy emphasis on religion, present in the Dark Brotherhood and Starfield, stories which otherwise had his complete say. It'd also explain his dismissiveness towards 4's story, since it wasn't entirely his own and it was pretty much a rush-job for a prequel/multiplayer game turned sequel.
TLDR: Fallout 76's
cheapness in story yet oddly developed map is Bethesda trying to make money of off work they already sank too many hours into. Fallout 76 was to be the original setting for a sequel to Fallout 3, and Fallout 4 was a prequel to 3, possibly with multiplayer. Emil was brought on during the last year of Fallout 4's development to make a story from pre-existing assets and make it fit as a sequel to Fallout 3, which was done as a consequence of fan reaction to another multiplayer prequel game they had just released: Elder Scrolls Online. It also helps explain its more light-hearted tone compared to 3 and NV, because it was never intended to be a mainline game. They had to make it so a game that had spent 7 years in development didn't just come out an immediate failure like Elder Scrolls Online was purported to be.
Fallout 4 likely suffered the exact same fate was Fallout 3 did, of mixing together to totally separate visions for its place in the chronology, and trying to make the original fit its new position in time. The original plan for 76 was scrapped due to Fallout 4 setting a new chronology, so rather than scrap it outright they recycled the original location for Fallout 4 into becoming what is now 76 and vice versa. I would even hazard to guess that elements of the original sequel to 3 were just shoved in haphazardly.
Ponder:
1) The "Institute" kidnap a genetically pure individual to serve as their leader (for no reason)
2) The "Institute" are conducting FEV experiments to create Super mutant replacements for "synths" (goes nowhere + no reason + FEV was a military thing, how do they have it?)
3) The "Institute" working with an outsider (Kellogg) is made a big deal out of despite them "canonically" braving a trek from West to East coast
4) You can join the "Institute", which otherwise has no logical basis given you can't actually alter course on anything it does (like you can in a certain other game)
5) Li is a member of the "Institute" despite the events of Fallout 3 giving her cause to shun such an openly malevolent organisation, and disliking the BoS despite their relative benevolence in 3, especially compared to her new organisation. Her description of why she left makes no sense given she left the BoS when it wasn't working on secret weapons projects, they were rather open in fact. The BoS also never used their control of Project Purity to nefarious ends. What was Li intending to do with Project Purity? Distribute free water? Like the BoS already did?
Dr. Li: "I was tired of being stepped on and used by the Brotherhood. First there was the water purification project, designed to freely benefit the entire Capital Wasteland. Even though the Brotherhood allowed it to be activated, they wanted to control it. Then there were other, more classified projects. Always using my work as weapons of war. I simply had enough."
7) The "Institute" are obsessed with the development of artificial lifeforms, but not even serving as a means to an end because no goal of objective is given

The BoS skip over Pennsylvania, New Jersey, New York, Rhode Island, Delaware, Connecticut, just to stop over in Massachusetts
9) Maddison Li somehow got here by herself despite needing escort out of Project Purity. The Commonwealth isn't even safe to do this since in 4 it's depicted as a similar lawless Wasteland as the rest.
In case it wasn't clear, I think the Institute were originally meant to be the Enclave as they would've been depicted in the original theoretical sequel to 3, but were then retrofitted to fit the assets and ideas made for a prequel instead, but a prequel that itself as now made into a sequel. Kidnapping a genetically pure human to serve as the Enclave's future leader could be the result of an AI wanting to avoid the issue Eden ran into with Autumn - they needed a human who was 100% indoctrinated yet competent to lead the Enclave. Similar to the player's influence in 76, it's possible the Enclave could've been changed from within. I think the player interactions with this hypothetical AI were similarly recycled into 4 with DiMA.
Fallout 4, like Fallout 3, is a result of combining a setting/location intended for one plot, but then made to do another, flipping character names and origins as required.
Fallout 76 is barren because the original basis for it was to serve as a sequel to 3. 3's possible OG date of 2246 would put F76 fittingly in 2276, 3 decades after. The main conflict, again, would've been Brotherhood of Steel vs Enclave, with the protagonist being a cryogenically awoken denizen of Vault 76, possibly meant to serve their purposes, or the plot would've been identical except Shaun would've been an indoctrinated servant/Vice President of the Enclave's AI.. 76's barren story is a consequence of its main story beats getting recycled into what would become Fallout 4, which in itself was also conceived as a prequel set in New York before being retrofitted as a sequel set in Boston. The settlement building stuff is likely also a product of being originally intended for multiplayer. The Minutemen/Gunners were probably the "Alliance/Horde" dynamic for the original multiplayer version of the Fallout which became "Fallout 4".