(Minor spoilers!)
Well, the straw hats are currently fighting against one of the four pirate emperors. Unless there are more surprises, there will be 3 more left after this one. And I don't see Luffy fighting to 'beat him' against Shanks. At most it will be a test, to see how much luffy has grown and to see if he really is worthy to be the one to find the One Piece. So I'd say Oda's getting pretty close now.
I'm still wondering what the One Piece actually is. It's impossible that it's simple treasure, considering the manga's themes so far. There have been some pretty nice theories though, like the One Piece having something to do with the ancient weapons. A means to destroy the holy land and the mountain it's on top of, therefore connecting all 4 seas (and also destroying fishman island as predicted by madam Shyarly since it's directly below the holy land. Which also requires the ship Noah to take all the fishmen away from there.) Though that's just a theory based on some vague clues.