You know what?
You have to see it like this. In East Blue we got 4 different nakama's. + Luffy for introducing but he doesn't count.
East Blue:
Zoro
Usopp
Sanji
Nami
Then we came into the Grand Line. There we found:
Grand Line:
Chopper
Robin
Franky
Brook
If this is some sort of, can't remember the word. Anyways it has some sort of way..
I think we can get 4 more people that live in the New World.
I do remember an image where we saw Hancock with 3 other characters. My bet goes to that image. And that those could be future members.
Now you all say, Hancock lives in the Grand Line. Yes you're right.
I am guessing different things now.
Since they introduced " protecting an island with you name thing " I think when the Goverment knows that Hancock has helped Luffy. Luffy says this is my turf!. While maybe " marry " Hancock in a DB way. And then joins him.
Also it's possible, and I think I am sure about this. Luffy meets Whitebeard. This Theory depends on wether he falls yes or no. If not, then Luffy tells he came into Impel Down with the help of Hancock.
The WG knows it and declares the war against the Kuja's. Then Whitebeards says it is his spot. So protecting Amazone Lily for any Goverment attack.
Also you have to known. It lies in the calm belt. I know the marines are able to sail over it and ignore the Sea Kings but still it's a very difficult attack spot, imo.
Or maybe, that we see her later in the story when he enters the New World. And she has chosen to stay with him. What could be possible.
But I think we gonna see Hancock back in the New World and then she will join.