you will notice the doctor in Camp MCcaran remarks that the Ncr will run out of food in around 10 years and won't be able to feed its rapidly growing population coupled with the fact that Ncr citizens are already burdened by extreme taxes the ncr government imposes on them they will struggle to afford food and water this would lead to food riots and armed resistance amongst the various Ncr towns and cities.The Ncr would probally have to pull their army out of
the mojave and abandon Hoover Dam,this would allow the legion time to recover considering the NCR ending for New Vegas is canon which im 90 percent sure it is,due to the ending for Fallout 2 being that of the Ncr's.So with the Ncr fighting its own people the legion would strike into Ncr territory also the Shi considering they still exist and weren't absorbed would also take advantage of the situation and attack.You could have all sorts of awesome scenarios with an Ncr civil war, such as maybe the grandson of an ex Enclave member who leads a rebel faction of fascist Ncr soldiers and civilians who hate mutants set up death camps etc to exterminate them paralleling ww2.You could pit the fascist Ncr against a communist Ncr political party paralleling soviet union vs Nazi Germany just a plot device for the brilliant developers at Obsidian to take a look at because i would very much like to see another game from you after Fallout 4 and note to the admins this is not fallout 4 speculation so please kindly do not lock thanks.