Something that many noted in the course of the Water Seven/Enies Lobby arc was that, at the end of it, we got the Third Person and Seventh Person (Usopp and Franky) but never got the Sixth Person (Robin). I think at the time most speculated on another runaway by Robin at a later point. I think this may not be completely wrong. Though it doesn't necessarily have to be a betrayal.
I think it's possible that Robin will become the "Sixth Person" after confronting what seems to be the most significant loose end of the Ohara flashback: The books in the lake. As Ohara burned, the scholars did what they could to save what books they could. They threw them in the lake, so many in fact that the lake was filled.
We're never told just what happened to those books. Some were probably too damaged by the lake water, but there had to be survivors. Also, given the status the Ohara Library possessed among scholars, it's probably safe to assume that some irreplacable volumes were there.
Yet, we know nothing about what happens to those books, or the Ohara Poneglyph for that matter, after the Buster Call. Did the World Government just destroy them? Did they keep them? If they kept them, will the crew possibly come across a library with the remnants of Ohara with in it?
If this occurs, I'm willing to bet that it will be the arc that signifies Robin's true joining. It would have her facing down the final shadow of Ohara's Buster Call: The destruction of all the history. If Robin were ever to learn of such a place there's a good chance she'd bee-line right for it. I could easily picture her seeing herself duty-bound to take those historical tomes from the hands of those who don't seem to understand their value.
And, really, would anybody be surprised at the possibility that Franky included a library/study in the Thousand Sunny's design?
But, as is always the case in these things, that's just my opinion. I could be wrong.