You could just have a group of Tilted Forum members who play
Second Life. This MMORPG allows people to have (for lack of a better phrase) a "second life". Everyone could walk/fly around and still be able to chat. Create or buy other objects etc and interact with each other.
It was a great concept when I beta tested it, alas I don't know what it's like now. This is probably the closest you're going to get to a 3D chat room or RPG. The members here just don't have the time neccessary to create an entire world - do you have any
idea how involved it is? I play
EVE: The Second Genesis and from what I understand was in development for a few
years before it was even in a beta-test phase.