> “Land area and objects are basically unlimited, since they run on the client"
I think this is very significant. The server-focused model of Second Life and OpenSim just makes things very expensive - after all, practically all our users have machines that can run physics on them (if they can run graphics, which they must!).
We would need different technology though. Unity is nice but it costs money and is closed source. We do have open source projects that run full 3D environments on the web, like BananaBread,
https://developer.mozilla.org/...
(which I have worked on). This could be tailored to run a virtual world, and would do the heavy work entirely clientside and use only free and open tools (zlib, as free as is possible). And it wouldn't need plugins, instead it uses WebGL and JavaScript.