There's a fantastic book called Snow Crash written by Neal Stephenson. It touches on a ton of subjects including history, linguistics, religion, and computer science. The main part of it that I love is the idea of the 'Metaverse'. Basically, there's a publicly accessible virtual world. Anyone with a computer can access it. When you log in, you're represented by an avatar. This avatar you have to either script yourself or purchase from some other place.
The history of it is that it was initially this blank place. There was nothing there. It was subdivided into plots of land, which you could purchase (or grab for free in the beginning). Within your plot of land, you could program in and model anything. In the rest of the world you can program and model most things, with some restrictions (you can't kill people in the public areas, and you can only kill people if it's been programmed in, etc.). The general idea is that everything is made by the users: it starts off a blank slate. Transportation, combat, avatars, everything is made by the users over time.
With the rise in accessibility of VR tech such as the Rift and some of the other newer options, this becomes a possibility. I don't have anywhere near the expertise to implement it though.
There are of course, some attempts to make this. Second Life, Habitat, Active Worlds, and There are a few that I found. They're in various stages of activity, with Second Life being far and away the most widely used. However, there are definitely a few differences between these attempts at 'metaverses' and a wider interpretation. For one, there is limited modeling. Secondly, it's difficult to feel like its a real world because of graphical limitations. Thirdly, the scripting environment, while good, isn't perfect. Ideally, the scripting will be easy enough to pick up that anyone can get into and learn coding from it, but complicated enough that you can literally do anything with it.
Photo: Alvord Desert