What plans are there for multiple totally separate game worlds?
There are a few ways I can think of; they could be implemented right into the server program, or distributed to several separate sets of server hardware- which would be more efficient.
Potentially, making the server software itself available (closed-source, of course; open-source is an entirely different discussion) might allow for some interesting systems.
Whether or not to have separate worlds is the real question- it's a legitimate decision to keep it centered on one world. On the other hand, separate worlds, especially when tools are in place to limit who can join (such as playtime requirements: "you must have spent 8 hours playing before you can join this world"), allow players to feel safer from griefers and such.
Other options, such as high-roleplaying servers, allow players to join in like-minded communities without overlapping with other groups (I.E.; no crowds of Urists muttering dwarven obscenities around unrelated Elven towns).
Finaly, there's the option for map parameters to be different in ways that don't work very well for consistent world maps; one map could be grassland punctuated by various other terrains, like the current one, while annother could be normally forest with clearings of various kinds, and annother could be a vast sea with many different islands; and each world could have easy access to a Ring. Of course, large-scale terrain shifts could render this point moot in the face of a large, varied map.