A crazy idea: you start with a spawn at (0, 0), with boundaries of (-2000, -2000) to (2000, 2000). After three months you move the spawn to (0, 4000), with new boundaries stretching from (-2000, -2000) to (2000, 6000). And then repeat, moving the spawn point and moving the boundaries.
So every 3 months you'll get a massive chunk of new terrain, new players will spawn right in the middle of it, but the older cities and structures will always be there, sitting casually to one side, waiting to be explored..
Too crazy?