From what I understand, Nami became a straw hat pirate to draw a map of the whole world. Doing so forces the implication that the whole world hasn't been traversed yet, in spite of the claims of Gol D. Roger the King of Pirates.
This leads me to believe that there is still a part of the world untraveled by any pirate. Because if there wasn't, then there wouldn't be a driving goal behind her character development.
So, can we really trust the current layout of the One Piece world? Or, has Oda made any announcements or claims that the world of One Piece as we know it is complete in its design?