Personally I have been considering the idea of a battle between a Northern Collection of states(FSA) and a southern collection of states(CSA), over control of DC and the Columbian Commonwealth. The complication is that DC itself is mainly run by high tech survivalists, due to the amount of radiation in DC proper, most people communities have to be very high tech to survive.
This radiation causes several problems, firstly it means that most large farms are impossible, due to the need for plentiful water, and the radiation combined with East Coast FEV experiments has produced a highly aggressive mutated strain of humans, who have highly variable appearances and fill the role of late game enemies. These mutants dwell in the parts of DC that matter, namely the vast spiders web of pre-war bunkers, due to the surface of DC being so blasted as to have destroyed it in all but its entirety. The Survivalists attempt to scavenge technology for use in production or replication, so that it can be sold to the outside world.
It is for this reason that the conflict arises. Namely that the CSA and FSA seek to control the region for two different reasons, the FSA wants to use it as resource extraction for a war with Ronto, and the CSA want to use it to become an industrial area, something the CSA with its focus on Agriculture lacks. Finally, the BOS have recently moved into the area in their quest to locate and secure the highly powerful weapons in DC, namely the unassembled legion of Liberty Primes, as well of the establishment of Columbia as a semi-independent vassal state. The knowledge of these robots is a late game secret and their capture is one of the FSA's aims, so as to wrap up its war with Ronto quickly. Similarly, you can aid a native faction of Power Armored descendants of parts of the Pre-War US Marine Corps, to secure the Independence of Columbia, these I would name the Regulators.
On the larger factions there are several points. The FSA is similar to the NCR however it is more industrial and probably more developed, however they are relatively uninterested in the rights of the DC Subjects, in many cases not granting them citizenship so as to be able to abuse them. So most of their advocates espouse the very real trade benefits, namely how they will be tied into luxuries unable to be had otherwise. They also have taken to dumping prisoners in DC to work as scavengers, similar to the Powder Gangers. Comparatively the CSA is deeply racist, having descended from segregated Pre-War Vaults, but are relatively interested in making the lives of its White wastelanders better, often at the expense of the large black population. The BOS are interested in making wastelanders lives better but in a detached paternalistic way, through the dearming of the region, and the annihilation of the mutants, which all parties want. They also want to use DC's technological prowess to fuel their war machine. Finally the Regulators, would be a relatively difficult path to follow, can be modelled into several different paths, some of those can be to model themselves after the surrounding factions, be that FSA, CSA, or BOS, or they can be moulded into their own group, that emphasise their own traditions, namely of being Wild West Type lawmen, only in 7ft tall heavily armored war machinery.
In terms of History, the FSA and CSA I think are pretty self-explanatory, but if anyone has questions they can ask me. But on the Regulators, they are at the start of the game in a bit of a decline, this is due to the fact that with the rise of Shipping in the FSA, on a large scale, something they developed due to the war with Ronto, their position as the Tollmen of DC has been threatened, and so at the start of the game they have lost a lot of control. However on the BOS, I dont actually think Beths lore is that bad, just that they need to be based in Raven Rock, not in the Pentagon.
With this setup I would also add in some minor factions, who can be moulded into larger groups based on your actions in the game. For example, a group of Christian Tribals named the Appalachians, for the area they reside in, who control the vital region of Shenandoah, which is the main food producing region for DC, and as such the rural CSA seeks to break it. Raiders would include escaped FSA prisoners, drug fuelled Raiders, a biker gang type that threatens the roads used to transport coal into Columbia. Similarly, there would be a variety of towns, such as Charlottesville, Richmond, and Baltimore. The towns in Virginia would be independent however the CSA would be attempting to take them over, due to the decline of the Regulators, who would be largely unable to oppose it. Comparatively, Baltimore would be the primary northern town, and its largely Black Population, are it would be a scavenging/industrial town. The furthest afield west you can go would be Charleston, which is currently facing a similar problem as Richmond and Charlottesville, where the CSA is attempting to gain control of it, however here it is more a case of the Mine owners wanting to take control of the town, and they happen to be CSA men.
But if anyone has questions on this fire away.
N.B. the BOS are more in line with the BOS from fo4, however do actually want to preserve technology.
