Depending on how much of continuity between maps you need, this may be pretty hard to implement. If you'd like two "neighbouring" (generated this way) maps to be seamlessly glueable, with roads, mountain ranges etc crossing the edge, then it's almost impossible. If two maps need to be just vaguely coherent (e.g. the original map is a coast running from east to west, then its extension to the west just needs to be a coast as well running from east anywhere else), then it's much easier to implement.