Hancock's island can be solved by any plot point that Oda chooses. The only insurmountable issue would be if Oda doesn't want it to happen.
Solve? So you admit there is a problem. Hehe. The point was that unless Oda creates a plot device to solve the problem with her island, she still has a 'problem' or 'issue' with her island. Which is what I said. On both accounts. According to your logic right there, pretty much anyone can join at any time, right? Buggy, Croc, Bon-Chan, Blackbeard, Sengoku…all it takes is a 'plot point that Oda chooses'. The only insurmountable issue would be if Oda doesn't want it to happen. o/