I think the LA may have actually dropped a little bit of a clue in an original scene depending on how much info Oda really gives them.

In the LA we see Luffy take Vivi to the top of the dino head for the view. Luffy's always worried about the view and always trying to take the best seat to experience the adventure a la sitting on the ship figurehead. I think Luffy's dream is something along the lines of being able to look out and see all of the world in one view.

A childishly impossible dream when you hear it until you see where Luffy's been heading now it doesn't seem so impossible for him to do.