The thing about the Fallout setting is that it covers such a vast area of land and of time that there are countless stories you could set in it without having to deviate too much as to make it a different series. While I was at work today, I thought up a few pitches for possible Fallout sequels/prequels/interquels;
Fallout Genesis. You were just a kid when your parents rushed you from your bed into the safety of Vault 51 and told you you wouldn’t be seeing your friends anymore. You’ve spent most of your childhood learning and training for the day when the Vault opens, and it’s up to your generation to re-create the world. It’s not supposed to happen for another 10 years, but one day the Overseer calls you into his office with some shocking news - the Vault door has opened early, and nobody can figure out how to shut it again. As the senior staff and security team are needed to guard the entrance against anything that might be alive out there, he asks you to venture out alone to scout the wastelands that once were your hometown, find a safe place for the people of the Vault to settle, and wipe out anyone or anything that might be a threat.
(Setting: A bombed-out city just a decade after the war. The Super Mutants don’t exist yet, but almost all surface dwellers are either ghouls or insane raiders, and friendly settlements are close to nonexistant. The player must choose which factions to befriend and which to make enemies of, providing for different main quest lines as in New Vegas.)
Fallout: Cascadia. After the defeat of Caesar’s Legion and the annexation of New Vegas, the NCR turned its attention to its northern border, the vast forests of the Pacific Northwest. Traders and wanderers have told tales of the “Republic of Cascadia”, a peaceful, technologically-minded enclave along the banks of Puget Sound, and President Meade decides to dispatch an official NCR delegation to establish diplomatic relations with them. You are an NCR ranger assigned to protect the delegation during the long hike up what once was I-5. Not long after fording the Columbia river, though, disaster strikes as the team is attacked and routed by creatures the NCR has never seen before; giant humanoids covered in hair from head to toe, taller than a Super Mutant, faster than a deathclaw, and smarter than a robobrain. Your allies torn to shreds by the beast, you find yourself alone in the woods, far from home, and with the fate of both Cascadia and the NCR in the balance.
(Setting: Western Washington, which in the 200 years since the war has returned to nature and regrown its vast forests. The city-state of Cascadia is technologically advanced and environmentally friendly, but lacks military might. Principal adversaries are the Sasquatch, who as the result of radiation have become intelligent and desire to take the human cities for themselves.)
Fallout: Luna. Vault L was the best-kept secret of the entire Vault project - constructed on the far side of the Moon, some of America’s best and brightest minds were sent there as the ultimate test of man’s ability to live on and colonize another world. With a uniquely-designed GECK, they even managed to terraform a large crater, creating within it a breatheable atmosphere, a temperate climate, and plants and animals capable of flourishing in the Moon’s low gravity. Luna City is a peaceful and comfortable place to live - until a massive alien mothership swings by and sets up a colony of its own. Now, with the aliens growing ever more menacing, and their visits to Earth itself growing more and more frequent, the Council chooses you to find a way to send them back to their own solar system.
(Setting: Just after Fallout 3, specifically the events of Mothership Zeta. Luna City and its surrounding Earthlike environs, as well as a terraformed alien biosphere on the Moon and the unterraformed area in between. It may also be possible for the player to briefly visit Earth, perhaps to acquire lost data from an old Enclave base. Going there would be a major quest chain as the player must find a way to survive in a gravity field six times more intense than the one he grew up in.)
Fallout: Ghoul. The earliest memory you have is the sound of thunder, the burning hot shockwave tearing at your flesh, and the blinding light of dozens of nukes toppling the tallest buildings in the world. You can’t remember who you were or what you did, but it doesn’t matter, because on that day you were born again. For God knows how long, you were little more than an animal - lurking in the darkness, feeding on anything or anyone that came too close to your lair. That all changed after your last meal stuck you with some kind of needle while you were eating his face, and now you’re starting to experience all kinds of new feelings. Sadness. Guilt. Remorse. Anger. Hatred. And beneath it all… hope. You don’t know where you’ll go, or who will accept you, or whether you can make a difference in this world, but you’re going to try.
(Setting: New York City, around the same time as Fallout 3. Parts of the city are ruins, while in others the skyscrapers still stand tall. Factions could include “Mole People” living in the ruins of the subway system, the Commonwealth, a group of patriots living on Liberty Island in a half-toppled Statue of Liberty, a group of aristocrats in Long Island who live in a gated community like the war never happened, and a Mafia family run by a ghoul from before the war, possibly having a history with the PC.)