Going back to the events defining Fisher Tiger's legend, I too read it as Tiger escaping his captives in Marijoa but when escaping he couldn’t bring himself to leave without helping the other slaves and consequently broke them free as well.
How I have been interpreting the history:
Fisher Tiger has only been to Marijoa once – while he was held as a slave.
Tiger freed the other slaves before escaping Marijoa.
Tiger never climbed the Red Line barehanded. That was a story manufactured by the government to villainise Tiger's actions completely and detract any sympathy away from his plight as a former slave by hiding that very fact from the public. The current World Government's position is built on twisting truths to suit their convenience. Ultimately, the government were unaware how powerful a pirate Tiger would eventually become and how their story would only add to the legend of Fisher Tiger.
With the freed slaves, Tiger returned to Fish-Man Island but only he entered to keep separation between the innocent Fish-Man citizen's and the former slaves labelled “criminals” (those Fish-Men were only able to return after Jinbe became a Shichibukai). Out of consideration for his King and to help Neptune understand his future actions, he informed Neptune of his intentions – to free slaves. Not the slaves located in Marijoa because they were already liberated but the slaves being transported aboard pirate, Marine and World Government ships. Tiger never intended to stay in Fish-Man Island after escaping Marijoa. Tiger does mention to Arlong and Jinbe that the Sun Pirates mission isn’t vengeance for what the humans did to them but rather a mission for freedom and liberty which is why he forbade any of the Sun Pirates from killing.
Tiger never told Neptune he was a slave or his story or asked for any help. He just informed Neptune of the mission he will begin pursuing which unfortunately went against the path Fish-Man Island wanted to travel on via Otohime’s dream of coexistence. Otohime has main character level perception and realised the conviction behind Tiger’s words and knew there was nothing she could do to change his perception of humans. Sure, Neptune did enquire if Tiger really did it after hearing the news concerning the incident on Marijoa but given Neptune’s unfamiliarity with Tiger’s past (until Jinbe wrote him a letter explaining it), that comment does not indicate that Tiger went to Marijoa after talking to Neptune about freeing slaves. The slave liberation from how I understand it, had taken place prior to Tiger returning to Fish-Man Island.
News of Tiger’s actions on Marijoa only came after this discussion with Neptune. This is when Fish-Man Island lauded Tiger as a hero for challenging the rulers of humanity. Those unwilling to leave Tiger to the Marines resolved to protect him and joined his ranks among the former Fish-Men slaves. Thus the Sun Pirates were formed.
Over the next three years, the Sun Pirates continued Tiger’s mission of freedom and liberty until Koala was returned home and Tiger betrayed by humans. Tiger, like Whitebeard, had their existences exaggerated by stories of their feats and capabilities. They were both in the end, just one man with one heart.
The lack of coverage surrounding the Marijoa incident was I thought due to Oda wishing to keep Marijoa a mystery. It could have also been to keep the information on the Donquixote Family hidden. Future focus may be given to the event when we learn more of Donquixote Mjosgard as his family did seem to own several Fish-Men pirates (former Sun Pirate members) and this may have included Tiger.
As for Hancock being a future Straw Hat Pirate nakama, there was a time I really wanted her to join - during the Marineford Arc when I was excited at the idea of Hancock or Jinbe becoming a future Straw Hat Pirate (especially if Hancock lost her Shichibukai status). But now, I feel it is better she remains one of Luffy's closest partnersLuffy's closest partner and not someone who serves under Luffy. Rather than define Hancock and Luffy's relationship as nakama, I would prefer a deeper connection. One in which their relationship is characterised by more intimate emotions. When Hancock does eventually come to assist Luffy, it won't be because she is bound to him by duty as a member of his alliance or because of her relation to him as a crew member but it will be because of a reason no one else shares - that she genuinely loves him. I know One Piece isn't focused on romance (unless you Oden and Toki) but as simplistic as Luffy is, he will eventually come to accept Hancock's feelings (I'm looking at you final chapter). Absence can make the heart grow fonder and I am sure that would apply to Luffy as well. Don't give up Hancock. Fight on!
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–- Update From New Post Merge ---
@Shiebs:
Do you guys think Yamato has a devil fruit?
if not will he get one?
I have been entertaining the idea that she will inherit Marco's Tori Tori no Mi, Model Phoenix ability.
