So back in Loguetown we got this panel of Nami posing in different outfits. One is extremely similar to Nico Robin's miss Sunday outfit, the other shares a lot in common with boa Hancock. So here's my theory, given that Robin became a straw hat, could it be possible that Hancock will become one as well?
Now this may seem crazy but here's a thing to keep in mind. Hancock's role in the story has gotten a lot more important as of recent. With coby and a fleet of marines(along with the SSG weapons most likely) hunting storming the shores of Amazon lily, things are not looking good. In addition, oda even made a comment that Hancock, Sabo, and vivi will be in a lot of trouble in the manga next year.
What this seems to be setting up is Hancock, Sabo, and Vivi being captured and Luffy having to save them. If this is the case, then we have a perfect opportunity for Hancock to join. Plus by then, both Zoro and Sanji should be strong enough to where she wouldn't upset the crew's power balance that a much.
But she has CoC? So did Rayleigh and Oden, didn't stop them from joining Roger.
But what about Amazon lily? Amazon lily doesn't seem that safe anymore after the marines attacked it. The Kuja might have to relocate to wano kuni(which will surely be under Luffy's control) after Luffy saves Hancock and all. If they do that, then the Kuja wouldn't really need Hancock to protect them anymore. Wano is strong enough as it is.
as for her role, I don't think she necessarily needs a specific role, she could just be a fighter.
So then that leaves us with her dream. What could her dream be? Well since she is in love with Luffy, it could be something simple like just helping Luffy fulfill his own dream. Or, since she was enslaved by the celestial dragons, she could want to help bring down the celestial dragons.
anyways I don't know if I'd say its exactly likely she will join, but I think she definitely has better Odds than Monet had back when that theory was popular.
