Since Nami overheard the coverstation between Luffy and Coby about having an sea stone on the bottom of the ship, and if Nami manage to have Franky instelled it; Do you think they will be able to leave Grand Line for South or North or West Blue? If Oda does that, I believe it would make the story reflexible reather then stiffly having the story take place in the Grand Line.
There might be creative islands outside the Grand Line like Ohara. But if Oda does that we may learn more about the One Piece world.