I'd have the Fort Irwin national training center become a thriving desert settlement, its a bout midway between LA and Vegas and located off of the long 15 (a decent ways off of it, admittedly).
Its over 1000 undeveloped square miles surrounding a small base, which would keep the masses of survivors away in the early days, and make it a real paid to find/reach for raiders later on.
I'd have the garrison and whatever unit was currently there for training, or at least those that stayed, all combine under the base command and work to do what they can to survive.
Using Fallout tech, they'd have a small contingent of vertibirds and whatever armored units and other vehicles they and the rotational training unit brought had, most of which would get cannibalized for parts, a few full companies worth of power armor (at least a full company of T-51b and the rest of older T45 suits), and a vault tec water reclamation system that pulls from the aquifer as well as recycling waste water.
Early on after the bombs fell, the base not having been targeted directly, they get orders to maintain order in the local area and lock down the base, but as comms quickly break down the commander quickly refocuses on gathering what supplies he can, having the foresight to empty out various stores of their stock of seeds and farming tools, and as the unrest gets worse they bring in what locals they can, including the workers at the nearest strip club.
In the early days, most die from the radiation despite the fully equipped hospital onbase, but enough people with skills survive to keep the community going.
By the time of Fallout, the community numbers around a thousand people, and using their water supply they run farms to keep themselves fed, but generally dont leave the expanse of their land other than occasional recon and supply runs, or patrols on the outskirts of the bases footprint.
After the destruction of the vats and the cathedral, several large waves of supermutants wander into Fort Irwin and are cut down, the perimeter guards noting them and guiding indirect fire while tanks come in to finish them off.
By the time of Fallout 2, they have trade routes up and running, have rebuffed contact with the brotherhood of steel who they deem 'weirdos', and have friendly relations with the NCR, with the NCR being left thinking they're more just an isolated farming community, and due to other priorities the NCR doesn't probe.
By the time of New Vegas, the NCR has come to realize the true scope of Fort Irwin, whose population has ballooned to around eight thousand.
Between the surviving vehicles, heavy weapons, and power armor, the Fort Irwin defenders are well armed enough to keep the NCR at bay, while the NCR could overwhelm them over time it's just not worth it, they dont raid, sell useful food, and their trade caravans usually include people who will repair items for pay, making them too useful to be worth fucking with, albiet the NCR does strongly desire to force Irwin to join.
Post New Vegas:
NCR Mojave- After defeating the Legion and House, the NCR spends a few years consolidating the Mojave fully into the NCR, Irwin becomes less important for trade but remains self sufficient.
House Mojave- The NCR and Legion both repulsed, House eventually builds strong trade relations with Irwin, eventually forming a defence pact against the NCR and other threats.
Player controlled Mojave- Irwin eventually becomes a strong ally of Vegas.
Legion Mojave- Irwin allies with the NCR as the legion slowly pushes foward along the long 15. Eventually the legion, tired of the stiff power armored resistance, gets several pre-war offroad vehicles and outfits them with armor... and one is loaded with a salvaged nuclear warhead. Irwin becomes a radioactive spot well off of the long 15.
As a faction, Irwin would be a well regimented society, with its faculties leaving it very able to regrow a population.
Starting around middle school age, in addition to their more regular education, their children would be taught to farm and live off the land, once they reach high school age they are given basic military training, the curriculum at all ages being an interesting mix of prewar education and more practical items, with their graduation gifts including combat armor and a rifle that matches their skill set, Bethesda laser rifle being the most common, Wattz laser rifle for marksmen, with conventional ammunition based weapons being hoarded for their more elite full time forces to conserve ammunition and parts that wear out.
Their most elite forces would be called OpFor, off of the opposing force of the old national training center, and they would function as a training cadre in addition to the heavy shock troops, being a heavy company sized element.
Only the best join OpFor, all have served in the settlements defence their entire adult life and are the most experienced who came up thru the ranks.
OpFor would have almost all of Irwins supply of T-51b power armor and full run of the armory, and be composed of four infantry platoons, a command element, a transport element with trucks to rapidly move them around, a heavy weapons platoon that will be either split among the other platoons or kept to defend an area depending on the circumstances, and most of the surviving tanks (two platoons of four), with the small number of helicopters and vertibirds available to them as needed.
Most of the time OpFor would be split into platoons with the heavy weapons platoon distributed among their ranks, and be positioned to guard the outlier areas of their territory supported by other units, with one full platoon always stationed on the road approach from Barstow incase raiders need to be kindly asked to fuck off.
They would have several more companies of infantry in T45 suits, well equipped and sitting in long established defensive positions, and every adult in the population would at least possess combat armor and a laser rifle incase of a massive attack.
At the time of Fallout their T51 suits would simply go to their most veteran troops, by Fallout 2, their population would start to baloon enough they reorganize their power armor and it becomes mostly restricted to the more veteran units, with the OpFor training cadre finally having the numbers to form a full company sized element.
By the time of New Vegas, they have:
OpFor: Company sized, standing around 230 at most times, the four main platoons are about 40 strong, with the remainder in the heavy weapons platoon, company headquarters platoon, and other support roles, the platoons are all broke down into squad and fireteam elements, but due to the unforgiving nature of the wastes they keep their most elite together so as to avoid loosing irreplaceable power armor.
Transport company: All of their various trucks and handful of aircraft, all electric and recharging their fusion cells from the bases reactors and solar power plants. Composed of the drivers and mechanics, day to day they repair things in town and ferry cargo around as needed, rarely do they leave Irwin, practically never to trade.
Able and Baker companies in T45 suits, these serve as the standard guard, and are the rank and file veterans of Irwin.
The power armor units compose the first battalion.
The companies of the second battalion are 'light' infantry in regular combat armor, they serve in a reserve function and have regular jobs in town, usually their rank and file will be composed of younger residents of Irwin and older residents who are too old to work in the fields, their jobs range from being the more visible guards for traders to interact with (noone is allowed inside the community proper), escorting the towns trade caravans, to simply walking the streets to keep the wildlife at bay.
Third battalion would be the training unit, consisting of all of the towns older teenagers and younger residents, those who wish to stay in the towns militia will transfer to second battalion or the transport company, the rest would train for a skilled job or to farm. In the event of a major attack, they could all gear up quickly.
Yes, I know, this seems purely military, but in the earlier parts of the timeline their forces would be much smaler. Most of them have day jobs, but the wasteland is fucking dangerous.
Now as for how to implement it ingame:
If done as a FONV mod, after Hoover Dam the long 15 would open up to travel again. Irwin would more or less be the more friendly boomers of the long 15, more friendly because they're way more likely to use loudspeakers to tell you to fuck off than shoot first, and most of their territory is way off of the long 15, they wouldnt have too many quests, and mostly you'd just see the occasional very well defended caravan, the NCR would have quests to attempt to sway some of the smaller settlements away from Irwins influence.
If done prior to FONV and the first battle of hoover dam but post FO2, they'd be allied with the desert rangers in the area, finding it far easier to help protect friendly settlements than have to deal with defense on their own territory.
In the FO2 era, they are the primary force in the area, and would have more of a presence as they scavenge for anything abandoned and useful.
In the FO era, they're the only force in the area.
Expansion wise, they dont really seek to take over land, they have lots of it.
Its mostly a fucking desert, but they have very deep wells to provide water, and over the years have built many cisterns so as to store as much water as possible, making them more than able to farm far more food than they could ever eat, leaving them with a surplus for trade.
