IRRC, I read somewhere that Obsidian asked about doing Area 51, and Bethesda prohibited it; (presumably they had plans for that one).
I kind of imagined Area 51 to be what Big Mountain was revealed to be, a top secret base were the government and various corporations like West Tek, Robco, General Atomics, and so on were working on all kinds of new and advanced technologies in various fields of science.
Heck I even had something like the Think Tank tough in my idea these were humanoid brain bots carrying some of the best minds of the base. They were even hardwired in their own way to focus on their work and never leave the base.
The player would also find a copy of the wreckage of the alien ship from FO3 as well as alien bodies, weapons and stuff from the mothership.
However it would be revealed that all of these are fake, they are sort of movie props created for a intelligence operations project to distract the American public and feed China false information to cover up what operations, research, and development the US military and its associates are really up to.
The beacon that replaced the crashed ship in FO3 would also be found here, revealed to be actually a hypnotic indoctrinator created as part of the project. While the short term plans were to bring confusion to the China command hierarchy the US also had another long term purpose for the project; once China had been defeated the US needed another threat to distract the general public from the resource shortage and energy crisis and keep the military economy running and state of martial law in place.
The government planned to place various transmitters at various locations where civilians would sooner or later run into them, the transmitter would then project the threat of an alien invasion into the minds of these individuals along with a science fiction scenario in which they are kidnapped by aliens, manage to escape and free other captives, and then confront the aliens and return home with stories about there being an alien threat out there that has been observing mankind since the last century and is planning an invasion of Earth, thus urging the government to prepare for that.
Under Bethesda we will only get more alien bullshit when they use Area 51. Imagine Fallout 5 being about the Core Region being invaded by aliens.
Beth was stretching it with the synths though I like them, but this would break the setting.
What next? Godzilla appeared in an Easter Egg so now Godzilla or giant lizards attack the mainland and dozens of manned Liberty Primes need to fight them?
Then there is Doctor Who which would require Bethesda to get the license from the BBC, so in Fallout 6 we become a companion of the Doctor and travel back in time to prevent the war which in secret has been set up by a Dalek-Cyberman alliance.