I do not own Fallout. If I did, this is how I'd do it.
Fallout: Orleans
Geography: Previous Factions
With FOO taking place after the events of FNV, major factions are still around. Revealed primarily via the Blue Waters who have been involved with just about everyone via trading or raiding, and a token representative or two in the setting, how well the factions have done depends on the outcomes of FO3 and FNV. Many factions have their own interest in the rise of Orleans, even if their influence is largely insignificant.
NCR
The NCR still exists, but its attention is devoted to issues closer to home. A water shortage and famine are making its pain of over-expansion worse, as is the ongoing conflict with the Legion and a low level conflict with the Blue Waters who are interfering with its nascent naval efforts along the coast. Between ongoing Blue Water piracy and its history with the Enclave, small wonder the NCR favors Napoleon (even if they truly liked the First Consul's republic more).
The token NCR representative would be a civilian who is now stuck at the Orleans Opera House. A tourist who came to experience Orleans culture, due to the Blue Water blockade of the NCR coast preventing her return home she has claimed the role of NCR ambassador to Orleans. An upper-class snob even back in the NCR, they do about as well as one might expect. Napoleon keeps her around for a token of diplomatic recognition from abroad.
The NCR's status depending on FNV:
In an NCR victory, the NCR is big, rich, and fat. The water and wealth of Vegas are propping up the nation, but the expansion has stopped as President Wait-and-See Oliver focuses on the drought and consolidation. Political reorganization is in the air, and Vegas may at long last become a state rather than annexed territory. Skirmishes with the Legion still occur, though they have all been well east of the Colorado.
In Mr. House/Independent Vegas, the NCR is big, poor, and starving. Famine grips the land, and the NCR's expansion to its north and south is little more than desperate grabs for resources. The only reason it doesn't risk severe unrest is that the desperate (and rich) are heading east to Vegas, gambling it all on a better life, while buying food from the city-state. Independent Vegas tolerates the refugees more than Mr. House, who has used Securitrons on refugee riots more than once.
In a Legion victory, the NCR is small, poor, and possibly dying. The Legion's advance westward is a desperate war that is only making the draught worse. Though the desert is a logistical nightmare for the Legion, stalling their advance and letting the NCR Rangers wreck havoc, it's unclear how the NCR can turn it around. At best, the Legion will tire and have internal difficulties before then… though don't count on it if Caesar lives.
Caesar's Legion
Despite its doubters, the Legion still exists as a force to be feared. Its cracks have begun to show, however- between its Caesar of the West and the Legate leading its expansion to the East, the Legion has functionally, if not officially, divided in two. Though its eastern advance is slowed
by the radioactive tornadoes of Texas, even from this distance the Legion has an interest. The Blue Waters are pirates to be crucified, and the drug-addicted Orleans deserve to be enslaved like the Profligates they are, but it's the Enclave and their reliance of technology that are seen as the weakest. That's who the Legion would like to win, so that they will have the easiest conquest of all when they inevitably arrive.
The Legion representative would be Frumentari who are, of all things, helping to train Orleans locals for the National Guard. While making a show of formal relations against more hated profligate factions, the Frumintari are actually trying to strengthen the Enclave enough to put up a better fight to weaken the others (or, better, win), so that the inevitable Legion invasion will be easier. Plus, they're starting to establish spies and informants in preparation for their own expected conquest in the future. The Enclave knows this, but keeps them around simply because the Legion's training regimen and discipline is simply that effective. Still, the Frumintari are watched by Enclave Marines… who more than match them in Drill Sergeant Nasty and physical toughness.
The Legion's status depending on FNV:
If the Legion won Hoover Dam, their main focus is the war in California. While Caesar rules from the capital of Vegas, the Western Legion tears into the NCR while the Eastern Legion continues to gather more slaves, territory, and weapons to feed into the western fight. Despite the logistical issues, the war is clearly favoring the Legion. Even as far away as Orleans, people fear what will happen when the Legion reaches the Pacific and turns back East. (Caesar Lanius is much the same, except he takes no capital, and leads the war in the West himself.)
If Vegas went independent, the Legion is diverted. Its conquest of Vegas stopped, and its efforts invade the NCR from the north struggling, its expansion to the north and east are a matter of least resistance. While expanding faster than it would if it were focused west, the difficulties of keeping the Legion(s) united are stressing even Caesar's skill, and nearing schism if he is dead.
If the NCR won, the Legion is truly devastated. The Western Legion barely holds the west as a rear-guard, facing NCR raids and little success in the north. Only the East sees expansion, but these are not enough to sustain both fronts. The Legion faces internal discontent for Caesar, an attempted coup of Legate Lanius, and a successful coup if both are dead, though it remains well capable of doing much damage as it lurches east.
Vegas
The City of Vegas, Queen of the West, has rumor even as far as here. The place where fortunes are won and lost every night. Only the richest can afford to visit from Orleans, and few return with as much. Vegas is the bell weather of the west, but too far away to matter. Only by sailing the Colorado with the permission of the Legion can anyone reach it. In return, Vegas barely knows or cares Orleans exists- too occupied at home, only Mr. House has a cursory interest in the mouth of the Mississippi.
The Vegas representative would be a gambler who broke the bank at all three casionos, and so won a trip to Orleans as a way to get them out of Vegas. As lucky as they are oblivious, they are convinced Orleans is the luckiest vacation yet, and act as total tourists. Can be found at Tourist Trap, not getting why it is bad.
Depending on endings…
NCR-Vegas is heavily taxed and supporting the nation, but doing well. It is trying to become a full state and get representation and voting rights, rather than an annexed territory
Legion-Vegas is the official (or unofficial, for Lanius) capital of the Legion. It is, in a word, dystopia, and the spirit of Vegas and excess put on a cross. The Gamorrah prostitutes were enslaved, obviously, and the casino turned into a giant breeding pen. The Ultra-Lux has become the Temple of Mars, with the Priestesses running it as a shrine to Caesar. The Tops was turned into a barracks with the ground floor turned into a gymnasium and public baths for soldiers.
Independent-Vegas is a big, independent non-entity. It follows a very mutually non-aggressive path, in which it doesn't pick fights and no one else can pick a fight with it. Few people know, or care, about the war in Orleans. Those who knew tended to think fondly of the First Consul's efforts, but find Napoleon's imperialistic centralization just too oppressive.
Mr. House's Vegas is the most active, sponsoring efforts to bring more people out of the wasteland and to spend their caps in Vegas. Orleans, while well outside of his influence, is seen as both an opportunity and future competition. Regardless of who controls it, Orleans will dominate the Mississippi, and river trade in and out of the continent. It will be key in selling Mr. House's technology. At the same time, though, the Enclave is a potential technological rival, the Orleans Empire might one day have expansionist designs to the west, and the Blue Waters have a monopolistic attitude towards business competitors. As Mr. House watches the war from his first satellites above, the prospect of an economic challenger is… interesting.
D.C.
D.C. and the capital wasteland has seen a population boom since the activation of the Aqua Purifier, and the rise of clean water has turned it into one of the most significant communities on the coast. Wastelanders from across the continent have headed towards D.C. in pursuit of the legendary Aqua Purae, and salvage efforts have intensified. Since Captain Supermutant approached the supermutant army and Talon Company and convinced them to leave the Capital Wasteland, the region has never been safer. Blue Water traders are regular visitors to trade for salvage, technology, and water.
The above is all true regardless of whether the modified-FEV was put in the purifier or not.
Assuming the Lone Wanderer followed the good route, the capital wasteland listed above is all that and more. Communities have risen, the Capital Wasteland Brotherhood of Steel is the organizer of a loose confederation of settlements securing and distributing the water, and with the departure of the supermutants and arrival of new migrants D.C. is finally recovering and repairing into civilized societies. In exchange for the Aqua Purae, the Blue Waters have brought all manner of trade goods and salvaged technology from abroad, at last stabilizing the Brotherhood's resources. Challenges and shortages persist, thanks to all the wastelanders migrating to it, but the region is an optimistic success of post-Fallout recovery.
D.C. is remembered by the Enclave as a disastrous defeat, and is the basis of many of Hans' reforms. After the defeat of Eden's purist plot, Col Autumn's Reformists, who would rather rule than kill the wasteland, came to power to run the Orleans campaign.
If the Lone Wanderer did President Eden's bidding, however… D.C. is a safe and significant community. An Enclave community. Despite the extensive losses to Liberty Prime and the loss of the Mobile Crawler, destroyed during the Brotherhood's assault, the modified-FEV had already won the conflict. Once the affects were recognized, the Brotherhood of Steel had no choice but to abandon D.C. and organized as large an evacuation as they could, and the local settlements that could not flee with them soon died. By the time the Enclave moved back in, there was no one left to resist- every settlement, every raider, every deathclaw and yao gai and radscorpion and ghoul had left or perished. Wastelanders, hearing of the safety but not the cause, still come. The lucky escape, or are able to sell their services to the Enclave for the rebuilding effort, and then buy water brought by the Blue Waters from outside the region. For the Enclave, and Vault 101 survivors, the wasteland has never been safer.
D.C. is remembered by the enclave as a Pyrrhic Victory, a success despite the military disaster, another one of which would destroy the Enclave. Leaving the Purist elements to live and reconstruct in D.C., the Reformists took control of the Orleans campaign when the Purist's initial invasion soon stalled and faltered. Governor Hans is intent on not repeating the same mistakes that turned D.C. into such an expensive victory, hence his efforts of diplomacy and local integration. There will be no usage of modified FEV in Orleans so long as he has power to stop it.
The Pitt
The Pitt has survived, and grown. With the trog cure finally being developed and spread, the city is at last regaining population self-sufficiency and stabilization. An industrial powerhouse by the standards of the wasteland, and a primary exporter of new ammo, the Pitt's favor has weight. Its nature, however, depends on the Lone Wanderer's choice.
If the Lone Wanderer sided with Asher and the Slavers, the Pitt is a mixture of authoritarian and visionary. While slavery is no longer needed, the slavers are not so easy to get rid of- Asher's enforcers, too strong to remove, have become the top of a brutal caste system that exploits and oppresses the weak. Slavery no longer exists, technically, and theoretically anyone can join the military, but woe to the workers who remain the lower class. Asher's reforms may falter, but his vision is still clear: The Pitt is a regional hegemon after having subdued Ronto, steadily expanding west to dominate the Ohio River Valley, and clearing the Ohio River as it does. Though it may be decades, Asher has visions of trading the Pitt's steel across the continent and through Orleans, accelerating the rebirth of the wasteland.
If the Lone Wanderer sided with the slaves, the Pitt is free and ineffectual. That might be a harsh description, but not inaccurate. When the raiders were driven out, they soon returned to raiding the city, and supporting the neighboring Ronto. While the free city of the Pitt defends itself and attracts both allies and escaped slaves from elsewhere, it's expansion has been much hindered. It also lacks the drive, and the forced labor, to do the reconstruction work to restore infrastructure- dreams of connecting to the Mississippi River or trading outside the region remain that. Despite the lack of stature, though, the Pitt has finally become a livable, even preferable, place- labor conditions are kind if not efficient, and while the Union that governs the city is preoccupied with remaining in power it does so through seeking favor rather than force. Escaped slaves and desperate people from across the region still seek the relative safety and security of the Pitt.
The Institute
Secretive, reclusive, and technologically advanced. Despite years of effort, this is all the well-traveled Blue Water traders know about the northern-most known civilization on the Atlantic Coast. A meritocracy in which the only merit of value is science, the scientist-elite live in The Institute and lord over the lesser-minded locals. While trade with the Blue Waters is virtually non-existent, little more than the occasional request for specialized pre-war electronics, they occasionally trade exceptionally advanced cybernetics or robotics technology that makes the effort a profit.
Author Note:
There were no carry-over mechanics in the Fallout games, so unless a video game version of FOO had a basic world-state creator for the above choices put in somehow, this is the sort of thing that would be pushed by the Dungeon Master during the narrative. Foreign powers aren't important as much as interesting head-nods to the past games- but they do serve the narrative theme of a wasteland of rising powers. Orleans is one among several Wasteland powers, which helps cast both Napoleon's ambition and the Enclave/Blue Water concerns that they will become marginal groups if they don't secure their own power bases to resist the rising states and proto-states.
Secret of the Bayou:
Planning this honestly felt like a game of Civilization. You have bunch of thematically unified civilizations, spread across the map, each in the early-game explore/expand phase of a classic 4X game. (The Explore, Expand, Exploit, and Exterminate model of Civilization games.) You could assign cultural traits (Napoleon as a cultural/aggressive civ leader), civilizational traits (Enclave xenophobia providing a strong penalty to integrating), unique buildings or units (Enclave vertibirds, Brotherhood bunkers, the Pittsburg stell press), and Wonders (the Shipyard, Project Purity, a Brahmin Baron exchange, etc.).
Throw in some other concepts from different Civ games and mods, like Hero Units from the Fall From Heaven mod (basically wonders in and of themselves, with exceptionally powerful stats and automatic experience growth: perfect for Liberty Prime or the various PC protagonists), Civ V's archeology system (to explore, exploit, and benefit from the ruins of the old war: say a choice between immediate weapons or long-term science gain), and a good system for dealing with Tribals as pseudo-barbarian units that can either be fought or recruited to bolster population.
I'm not going to be the one to do it, but just saying. You could make a decent Civ mod off of Fallout.