So I'm thinking of going through with my idea of a Fallout roleplay. It will have two parts.
What I'm thinking, for the first part, it starts in the Southeast, much like 3. We'll be playing along the same similar plotline: You have the option to be a Vault outcast (2 spots), a generic Settler (4 Spots), a Raider (3 Spots), or a member of the Brotherhood of Steel (3 spots). I may or may not have that many people, seeing as it's a bit more than I'm equipped to work with, since the only roleplay I've GM'd before had 7 people. But we'll all start out in our respective places, like the Vault Dwellers will all come from the same Vault, but it doesn't necessarily have to be 101. The Brotherhood of Steel will start out as recruits at the Citadel, and the Settlers could be from anywhere, like Rivet City or Megaton. Raiders...They're Raiders. Eventually, all of our characters (who survive) will meet up at the GNR building and meet Three Dog, because I love that man and I have to put him in. He'll tell them about Project Purity and how they have to get the scientists of the Brotherhood of Steel and Rivet City to finish the project. When they finish getting it partially operational, the Enclave will return, much like in the game. From there on out we have the option to fight for either the Enclave or the Brotherhood, so it's generally up for the participants to determine the ending.
As for the Raiders, you can do whatever the Hell you want: Raise Hell, attack caravans, raid settlements, etc.