notice that in the colored picture seen on the latter part of chapter 634 in which the straw hats are making the poses of their life, shirahoshi is included. is this an indication that shirahoshi will be part of the crew?
in my own opinion, yes! shirahoshi is a great addition to the crew basically because of her ability to summon sea kings. with that, they may pass the calm belt which lies deep the sea (correct me if im wrong) where sea creatures and sea kings lies. besides, the prediction said that there will be someone who will be guiding shirahoshi, it does not said that there will be someone to guide the princess meaning luffy's guidance onto shirahoshi does not end in fishman island alone. to do this, shirahoshi must join the crew. many will ask. how on earth will shirahoshi join the crew? she's too big, if you'll remember, there is somthing that can make big things shrink or something like that, im not quite sure but that will be the key. they may not need a helmsman but look what they will have with shirahoshi, sea kings pulling over sunny.
well, these are all assumptions and no one would ever know what oda is up to, one piece is simply the best.