http://www.theengineeringguild.co.uk/ the famous Oblivion/Fallout3 quest-modder has been talking about this whole situation, with the endings...
(I remember playing FO3 past the ending with his continue-game mod, long before the DLC for that game was released.)
But he's not sure in which direction they're going to go with this game's DLC!
Spoiler So it's challenging for him to write his mod(s).
Like - suppose The Legion was victorious in your game...?
So instead of 1 mod you'd need to do maybe 3.
He has already done a join-the-NCR mod, now he's talking about something for the YesMan players that will allow you to run Vegas after the game ends...
Anyway, I wonder how the official DLC will continue the game, or will it? Do we know that for sure?
L8R