Remember the background story of Fallout ! now, How would you see the 'first few steps' of Fallout?
Here is my version:
1) creation of character
2) Start in a Vault ( probably a few starting points located over the map to spread the gamers IF too much lag when too many people located at the same place )
3) While in the Vault, selection of your class/skills etc etc, probably mixed with small easy missions to learn how to interact with the UI
4) Story line will advise us somehow that 'something' going on out of the Vault and that we should investigate.
5) Discovering of the surroundings
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Now i do wonder if people will be able to create their own 'gang' with the possibility to literary own a town bringing business to it and so on. A MMO is also about business, crafting skills... don t you think ?