I just can't help but be amazed by the transformation Oda has applied to Franky's character in this arc. Just look at the complete turn around he has had since he was first introduced.
Just think about how you felt reading chapter 229 through to 333, and how much hatred & disgust you had for this guy. At that point I was thinking "right, we definitely have this arcs Crocodile or Eneru", this guy & his cronies had caused a major rift within' the Strawhats, turned nakama against each other and at that point I thought, there'd be no way this guy couldn't be anything but! a sinister, twisted bastard who is absolutely unredeemable.
But look a further 20 or so chapters ahead & I think the world of this guy, he's really quite unique & has a very intriguing past to him, not to mention a very human side also. So whether he joins or not, it doesn't really matter guessing at this point… as Oda will decide this & no one else. But I just can't shake the feeling about this guy, he's someone who feels somewhat integral to the continuation of the Strawhats journey be it that he becomes apart of the crew or he helps build their new ship, I just feel this character won't simply slip into obscurity & fade away when the crew leaves W7, he'll remain on, in some form.