It would be more then just a little work. People would also complain they can't do the quests. One reason I see for wanting it is so they can finish quests they could not before the ending. If Legion take over pretty much every quest in the game can't be done.
Very true.
The way New Vegas stands at the moment there are a total of 27 ending segments, 26 of which you can unlock during your playthrough. Of the 27 you are guaranteed to see 1 of them. Now on top of that each segment has anywhere between 4 and 13 variation to it.
The two ending segments that you will see one of depends on your characters gender and includes the four main factions and what your character karma was at the end of the game. All of this would NEED to be taken into account for any attempted post ending gameworld if they want that gameworld to be seamless with how you played the game. If they just dump you back out into the Mojave afterwards there will be a lot of people complaining bitterly because their action ingame are completely ignored in the post-ending gameworld.