I don't think hancock will join the crew although I do hope she stops by to visit the 1000 sunny just so we can see Sanji's reaction on Luffy nabbing someone with the title of the most beautiful woman in the one piece world with his wonderful idiot savant powers. Which is assured to be absolutely priceless.
It'd be really funny though if after the time jump to show off Luffy new found maturity we see a pregnant Hancock or a little baby post time jump ala Gohan.
Although I really don't think that'll happen because half of luffy's purpose is to move forward and introducing a child would imply that he's settling down or at least going to remain in static location and his overall ambition is to move forward and it'd be kind of irresponsible for him father a kid only to leave it behind.