I think it would be cool if the next fallout took place a year before the war, and after the bombs fall the player character slowly turns into a ghoul.
This way the game could conceivably leap frog through the years ala Vampire the Masquerade.
Bethesda have said that all Fallout games are to occur chronologically after the previous one. Chances are though that it'll still look like the bombs fell a few weeks ago, instead of a couple of centuries ago.
So this would be the plot of this game, one year before the war. 364 day and sometime on October 23, 2077. Two hours of nuclear armageddon.
A game before the war would not be Fallout.
"The end of the world occurred pretty much as we had predicted; too many humans and not enough space or resources to go around. The details are trivial and pointless, the reasons as always, purely human ones."