As for Rebecca joining, or anyone joining; The chances of anyone joining this arc is slim. And I mean joining the Flag ship. It's obvious other crew will be joining under Luffy, such as the Happou Navy, but as for individuals in the main crew; no. Barto is just too violent and evil. Bellamy killed Marry and is going to need a lot of redemption points to even be considered. Rebecca on the other hand fits the criteria for a straw hat nearly perfectly. Sad past, outcast in her society, leadership type position. The only problem is she is just too tied to Dressrosa and her dream involves Dressrosa. There is no reason to travel with Luffy and co, unless she pulls a Nami, who had a dream before the takeover of her island and had to abandon that dream for the dream of Freedom.