I still don't get why everyone wants to make a huge online server. This will fail, I can guarantee this. Why not make a coop Fallout game for 2 - 4 players? Nobody did this yet; it would be something new, it would be A LOT easier to make, it allows for more unique and new content that would never work in a MMO game due to player abuse and so much more.
Take a city of your choice - like Phoenix - write down a Fallout campaign, then create it with a bunch of players in mind. Add a few scripting mechanics to create a better coop experience (dialog screen for everyone, all players are always in the same group, nobody can leave a location if not everyone is in the exit grid, player respawn on doctors or some other form of respawn to make the game less harsh if wanted, WORKING TURN BASED COMBAT, a skill system true to the original game - because the ultimate skill balance in such a coop game isn't as important as in an MMO, etc.), release it as full game to the public and it will be super cool.
If I had more time, I would start such a thing and not a new huge online server, which will suffer under not enough players anyway. Damn, this can even be released in chapters without any problem. Do content for 1 or 2 hours first to see how people accept it, then improve on the content and do more of it. In any case such a thing will bring more internet fame than YAFS... Yet Another FOnline Server.
Hell, I would even love to work just on a template game, which can be used by other people to create their own FOnline coop campaigns with not much scripting knowledge needed. Such a thing is badly needed, imo.