A challenge, but certainly feasible. They'd have to split the planet up into instanced zones, and the map data would have to be hosted serverside, streaming to the end user as they move around the world and stored in a temp cache on your local machine. Depending on the quality of the map, I'd say about 100tb+, which for a project like this, is nothing. Imagine the possibilities!
Obviously it'd require a permanent connection to stream the content, and the combat would have to be turn based or slow a-la WoW, but with the right motivation and finances, this could be a very interesting project to follow.
I support this 100%, it's about time the gaming industry took a fucking huge leap again.