Essentially, set up the servers like an ice-cube tray and as each server fills up, it spills over into the next server, and divide it all into game regions. One huge benefit of this would be to allow there to always be a steady population of people leveling in every zone for group questing, etc.
Example: if I went to Borean Tundra right now, there may be 1 person questing there on Auchindoun, and maybe 5 on Maeiv, and 50 on Tichondrius. Under this methodology, there would be 56, up until an arbitrary cut-off. And if the cut-off is 100, I would have it start transferring people to a second zone instance at around ~70 so the 101st guy isn’t off by himself. The key would be to make it subtle, with no load-screen or anything. With phasing technology it should not be a problem.