The difficulty is that a product marketed in a way as to mimic a more mainstream MMO is relying heavily on some of the traditional social features of such. You NEED to be able to find your friends and communicate with them across worlds since the notions of group/social play and the notions of huge/open worlds are both key to the principals of the genre. The solution to this might be to expand the use of the ‘social’ features (friends lists, clans, etc) to actually alter the way that chat works. Make voice communication, party, and general chat ‘local’, but allow for transcontinental clan or ‘linked friend’ conversation – perhaps not in real time, but as a message system.
These ideas are just that. I dont have time to think them through in detail the noo, but worth coming back to.