No one advances in the game industry by following the same forumla. The main reason new vegas failed to get the plaudits Fallout 3 received was it didn't have the same "shock" to it. Yes, they made some pretty cool changes but fundamentally it was the same game, like an expansion pack. It would be wise of them in my opinion to consider making a prequel showing major factions were formed considering how many users like myself have never and will likely never play fallout 1 or 2.
That said I don't think you're wrong, they will likely just keep doing what they are doing. But it would be impressive if they did something that surprising, and the last time this series experienced fundamental change it won game of the year by many.
Like Sebor said, they're not going to remake FO1 & 2, just to show you how the NCR and other factions from those games we later see in New Vegas, grew. As given, Call of Duty hasn't changed to extreme degrees in years, yet it has a magnitude of followers, just waiting to buy up every game that comes out in the series. You say NV was like a "FO3 add-on", yet don't realize how different FO3 and NV are from each other, in every aspect of each game. They might have shared the same engine, but how they played and were done, were very different from one another. The amount of RPG depth put into NV, was a hell of a lot more than FO3, which was really nothing more than a FPS w/minor RPG elements added onto it. A lot of people overlooked that fact (including reviewers), among others, or just didn't care because of one reason or another, mostly because FO3 was their introduction to the series, and didn't care much for what NV added.
What Bethesda can do with FO4, is show us how
new factions (created from their own imagination,) grew/grow, and form to be what they will be in later installments.