[Suzerain of Sheol]

Well, on that note, you touch on another difficulty: that of having too many hands in the pie, so to speak. If the goal is to create a cohesive, consistent secondary world, the amount of input has to be limited, otherwise it'll end up being taken in too many contrary directions to hold up verisimilitude.

On your other point, I just don't think the work such a thing would involve would even be feasible. Imagine if a race was added that had four arms. That would require every single item of clothing to have poses added to it specifically for that body type. That's a mind-boggling amount of work. And limiting yourself to only what can be added to humanoid avatars... you end up with more or less what we have.