I feel it would be quite free-form, but as far as PK-protection goes... I think it wouldn't be too absurd to think there are high-risk/low-risk/no-risk zones.  There would be a heavy emphasis on roleplaying, for everybody, and staying in character.  We could even make it a rule, keeping anything real-life related to private conversation.  The risk zones would likely be in a "low-risk," only players with similar levels can target one another, but high-risk would be anybody can attack anybody else.

Plot in-game is quite tricky... considering the game isn't really... dangerous.  Nor is it the true focal point - it's more a medium for our characters through which to progress their own development.  (Did that sound as good as it did in my head? XD)