There's plenty of speculation going on about Fallout 4 which is fine but in my opinion after F:NV I'm more interested in Obsidian's take on the franchise at the moment and their next Fallout spin-off in the core-region... especially if it were to follow-on from the events of the last game. From my perspective I believe that irrespective of the ending you choose at Hoover Dam that in the end neither the Legion or the NCR can hold onto the dam in the long run and as a consequence New Vegas and it's people will eventually prevail in the three-way-tie to secure their independence from both factions. As a result the Legion and the NCR will be forced to withdraw from the region which will prove highly beneficial to the NCR and disastrous for the Legion:
As a result of failing to secure the dam and New Vegas the war effort will eventually bankrupt the Legion's economy which will put an end to any dreams of westward expansion and after Caesar's death - in the absence of any strong leadership and Caesar's teachings many internal troubles quickly lead to widespread disillusionment and open rebellion. The NCR will then seek to exploit these vulnerabilities to 'divide and conquer' Caesar's Legion by secretly funding and arming many of the rebels in order to put an end to the 'Legion menace' once and for all - signalling the final death knell of the Legion. The NCR had hoped however that the implosion of Caesar's Legion would allow them to recapture the dam and New Vegas but the political unpopularity of the war coupled with financial troubles and the election of newly elected President Hanlan quickly put an end to these imperial ambitions for the time being at least.
Although many might see them as a rather one note faction my first inclination would be for Obsidian to revisit the Legion and chronicle their decline, break-up & transformation after the events of Hoover Dam taking us deep into the heart of the former Four States Commonwealth:
Where the remaining loyalists devoted to the ideals of Caesar under the leadership of a 'Constantine' or 'Augustus' figure were driven from their lands to the eastern-most fringes of their former territory where they fortify their position (a 'Byzantine' Caesar's Legion anyone?) against many of the newly independent factions born out of the former Legion empire that are now all vying for control of the region; a vast merciless expanse that stretches from the 'Byzantines' all the way to the banks of the Colorado river on the shores of the New Vegas territory [The majority of the game area itself I imagine would be centered right in the middle of all four states containing parts of Arizona, Utah, Colorado and New Mexico with fast travels to game hubs(HQs) of the main factions].
Main factions:
1. Byzantine Legionnaires: Reformed and nuanced by the actions of the courier under the new leadership the Legion aim to recapture their past glory by quelling the uprisings and reunifying the former Legion tribes by taking and applying many lessons learned from the past (ancient history and recent events) that lead to the collapse of their empire: the Legion reformists now revere the burned man as a martyr god for his resurrection and conquer of the White Legs and have lead the Legion into a second golden 'Byzantine' age from a rather sorry fate akin to the Khans.
2. Ottoman Turks: Meanwhile an Ottoman turks inspired faction founded by former Khans (formerly Legion slaves) and consisting of bands of local raider tribes that had rediscovered their tribal roots after being absorbed into the Legion or enslaved are amassing at the gates to raise 'New Byzantium' to the ground to make sure the days of Caesar's Legion would not return and put an end to the greatest threat to undermine their independence.
3. American Pioneers: pioneer-era themed faction with civil-war themed clothes and weapons: After visiting the old world wonder that is New Vegas and being exposed to new ideas but being opposed to the debauchery and reliance on high-technology a group of former Legion 'idealists' banded together with refugees from New Canaan and disenfranchised NCR ranger veterans, troopers and civilians, etc inspired by the founding principles of the NCR and the simpler time of the pioneer era to lead the first uprising against the Legion believing in the cause of freedom and democracy. Vehemently opposed to the slaving and raiding practices of the other factions and intent on creating their own democratic republic inspired by more traditional old-world values of the pioneer days and the early days of the NCR - free of corruption and tyranny. Though they receive financial support from the NCR who believe they are settling the pioneer on their behalf and want to be admitted into the union... but the leaders of this faction have other plans.
4. American Indians: New Vegas-backed American-indian themed rehabilitated tribals faction. Friendly trade partners of the Civil-war-themed faction though specialize in Energy Weapons so mostly trade food and basic supplies. New Vegas wants to help them to pioneer the East in their imageā¦ modelled on New Vegas-style old world values with the intension of creating a capitalistic society obsessed with old world luxury so as to bring in a whole new set of customers who will consume Hoover Dam's hydro-electric power and flock to the strip in droves. New Vegas is looking to rebuild its fortunes in the east as the NCR and it's citizens have fallen on hard times. New Vegas is initially concerned with securing monopolies on all trade routes to the East and securing the region's resources so as to maintain their monopoly on electricity and clean water with the additional goals of searching for new allies and trading partners and liquidating any potential enemies of New Vegas before they pose a serious threat; however New Reno and other gangsters and hustlers have their own designs on the region and may have gotten there first.
Minor faction:
5. Ghoulslingers: With the power vacuum that has opened up in the region marauding bands of Mexican 'ghoulslinglers' are invading and settling the region after hearing word of the legion's decline. These banditos make easy pray of the fractured former legion settlers picking over their camps like vultures - driving many of them right back into the arms of the 'Byzantine' Legion's camps. The player must help the different factions grow and become self-sufficient - though which ones and how successfully would be up to you.