OH GOD YES! THIS is why I still love One Piece after all this time, even when we think we have things figured out about the story, Oda SLAMS Us with something this deliciously epic!
EVERY Crew member (sans Robin) has completely declined to Join and then actually DOES Join at the end of the arc, for the first time, someone accepts THE MOMENT he is asked XDD it's something different!
As far as what happens in the future, I can't predict any one thing, I have like 10 predictions in me and I can't choose one that fits!
Mr. Oda, You've done it again Sir!
Anyway, Like I said, it's WAY to hard to accurately predict what'll happen at this point.
Either he is:
A: An Actual new member of the Crew and will stay as such
B: A member who joined now, but will unjoin (or fully oppose the Strawhats) for some reason, but will then Rejoin at the end and leave with them
C: A Vivi-style member, who can't/doesn't want to leave and stays
D: He turns out to have been a villan all along (IE Gaining their trust to do something dastardly, like change them into ghosts too) and Luffy ends up having to beat him before leaving the Triangle
E: He is their Friend for a while, then something Causes him to turn bad and he either comes back to his senses and helps them escape, comes back and fully joins, or stays Evil and is beaten
SO MANY POSIBILITIES!! I LOVE YOU ODA! FATHER MY CHILDREN!!!