I liked elements of your plot outline, but some things didn't 'fit' really, especially the Overseers involvment and his control over Talon Company etc.
I'll just outline a few of my ideas.
Premise:
Due to numerous missile silos and defense grid bunkers situated in the DC area, the Capital Wasteland was hit with a particuarly ferocious barrage of warheads, most importantly, radiation bombs to ensure no one survives the aftermath to activate the many defenses.
Now, this is more of an example, there has to be something important and crucial to the betterment, OR destruction of the wastes, situated within DC, and the radiation provides a barrier to claiming such a thing, and also gives excess liberty to design some particuarly fresh and new enemies.
The Brotherhood of steel, fortified in a nearby irradiated vault, are in the process, and have to some extent developed radiation resistant technologies, that allow them two levels into the vault, without harmful side effects. The rest of the vault is sealed off, and a small settlment, punctuated with towers ad fences is erected outside to manage the large size of the chapter.
The BoS seek to travel into the DC areas, brave the dangers (detailed later) take the technoligy found there and migrate east, to save the BoS central force, in a loosing battle against NCR. The BoS are prepared to use any tactics necisary, and will take any tech they want or need, by force as they travel through the wasteland.
The BoS also have a forward camp on the river across DC (I have limited knowledge on American geography, sorry, you know the one just ahead of the Super Duper mart) where they collect and transport water to thier vault hq.
The BoS originally comandeered the vault to obtain a GECK, but have subsequentley realised there isn't one in vault 101, and move on, to your own vault 102 (namesake) and have been shadowing and observing the entrance for around a week.
The overseer of the vault realises this, and sees action must take place. One night, an attempted covert escape attempt is made, Vault citizens are escorted out by security, carrying as many supplies as possible, leading to an ambush by waiting BoS troopers, which leads to a crossfire between them, and the Outcasts.
The Outcasts split with the BoS over how to handle issues in the wastes, the Outcasts wanted to use the DC treasures to command power and unite the wasteland, to prosper, and develop a coherent city free from threats, but the BoS see no need to help the 'tribals'.
The game starts in the firefight, and Outcasts rescue you, and a handful of surviving vault dwellers, GECK in hand, to Fort Indipendance, where, as un-irradiated intelligent beings, you are given the option to become profficent in thier ways, and one can have custom special pre sets avalible depending on thier class choice. I.E Paladin, Medic etc.
Other factions:
Talon Company;
A large force of well armed troopers from NCR, formerley a mercenary branch, after they began to grow in power, thier leaders were hunted and murdered, and thier numbers reduced. In response, they fled and established a base in the outer-wastes, claiming whatever rescources they can, and amassing an army for a push on DC.
Mutants;
Super Mutants are abundant, remnants from the Masters army, and the radiation in thier habitat of DC, has twisted them into unspeakable beings, as terrible experiments, led by a mysterious cult of humans, allied with the mutants, fuse together life forms in sickening ways.
Ghouls number in the hundreds, the lap dogs of the mutants, and most are driven feral, except a few, who are absolved into the cult's ranks and never seen again.
Abominations (think Warcraft III)
A result of thr Cult's experiments, lumbering husks of sewn together meat, cybernetically programmed to follow the Mutants bidding.
Other Mutants would include malformed versions of Greek mythical beasts, roaming the city of thier own accord, answerable to no one, and known, as the Titans.
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Rivet City:
A sprawling population resides within a beached aircraft carrier, and a wall forming a vast corridor connects it to the nearby metro, wherein more of the popualation reside. Despite the cities sucsess, they are in immediate threat of the BoS.
Megaton:
A large slum, ungaurded, but surviving many years due to the intergration of raiders, which has gaurenteed the town quarter from the many overlapping raiding groups, who all have an interest in keeping the town alive, due to it's extrmley large Brahmin herd, only handlable by Megatons well versed farmers.
Paradise Falls:
A slaver metropalis, formerely a Followers of The Apocalypse camp, slavers took it over, and have since used the Followers knowledge to prosper, and build a sprawling slaver setllement, seeing the BoS presence in the area, they want to expand and take Megaton, and Canterbury commons, and merge the towns to hold the line against the invaders.
Canterbury commons:
A ruined town, with one exit, and one entrance, the innter area is heavily walled, using rescources from the crumbling suburbia, and within lies a vast wealth of traders and rescources, kept safe due to Rileys Rangers, a former NCR elite squad, who, after service migrated to the wastes to improve life, after having comitted so much bad under NCR.
The Many:
(Apolagies for similarites to The Master)
A secretive clan of humanoids settled in a series of mountain caverns and caves, the leader of which was exposed to FEV in an Enclave experiment, and was merged with an insect, wherein he took upon hive mind characteristics.
They have brought many under thier banner using cybernetic implants and radiation exposure, and seek the DC treasures to expand and gain comprehensive control of the wastes.
The Plot:
After being taken in by the Outcasts, and after a number of years in which the PC becomes profficent in the ways of whichever class they specified (or custom ofc) the Outcasts draft your service in exchange for thier rescue many years ago, to help them in thier quest to find the warheads.
From hereon in, the PC can aid the Outcasts, other factions etc, to restore order.
Ehm, very, very brief outline, obv if I were to write every branch and character in detail i'd be here for a year...but yeah, not as good as Dutch Ghost's but have fun reading