I don't like Vivi joining at some point after the Reverie mainly because she made a firm decision on where her motivations lie. Not for personal freedom and ambitions on sea, but serving and protecting her country. I want that to stick since her speech at the end of Alabasta rounded off her entire character arc in the Alabasta saga with trying to stop the civil war and be an efficient princess. I do not want that beautiful climax cheapened by having her join in full anyways, even if it has been years since then. This is very different from Jinbe saying "I've got some business to finish with my country right now, but I'll regroup with you guys at some point."
I am very sure that Oda can make Vivi into a proficient fighter by making up events that happened for incidental timeskip training and reinvigorating her weapons (man, those peacock slashers were so lame in both design and effectiveness). He already did the same for Nami and Usopp, and he is STILL milking new knowledge and abilities Luffy supposedly learned during the timeskip. She could even get a Devil Fruit for all we know.
However, I can't see her having any good reason for coming back to the crew unless it is to help her country if shenanigans ensue at the Reverie when Cobra starts blabbing about Poneglyphs and the Void Century. And that is exactly the problem. She would only join to help her country. Not for her own freedom or dream, not to hang out with the crew again that she loved as much as a family, but primarily as a means to an end for helping her country. And that feels so… antithetical to what defines a Straw Hat. Where they are all in this together to the point that they will sacrifice their lives/dreams to protect one another and have fun adventuring for earnest romance.
Remember earlier this arc when Jinbe was pushed to join the Straw Hat crew by the Sun Pirates because they could tell how much he really liked Luffy? They wanted to reward him for all of the sacrifices he made for his country. Sacrifices that were not entirely of his own free will like Vivi, but as necessary measures to support Fishman Island's standing against bigotry. That is what fits a Straw Hat's usual unwillingness to join the crew at first, forced obligations.
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Jinbe still needs his own personal flashback since Fishman Island's flashback was neither centric around him or defined a clear individual aspiration for him. He can get one either right after this arc ends or during Wano Country since Robby brings up how he might have connections there because of his attire and relationship with Ace (who has been in Wano Country and Oda said he wants to right a story about Ace there). But my point about Jinbe joining the Straw Hats out of selfishness rather than selflessness in contrast to Vivi's willful allegiance to Alabasta still stands.
Or how about a scene all the way back right after Don Krieg is defeated in the Baratie arc. Luffy asks Sanji to join his crew again and the latter refuses again, but Sanji still enthusiastically geeks out about All Blue's legend while Zeff is watching in the distance knowing that his heart's yearning lies more with the ocean that fulfilling an imaginary debt at his restaurant. I firmly believe that one scene can act as a microcosm encapsulating everything that defines the Straw Hat joining formula literally and figuratively.
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All of that comes right after doubting Luffy and Zoro putting their convictions above their survival throughout the arc when fighting against Don Krieg and Mihawk. Can you imagine Vivi doing anything like that this far into the story? Because all I can see instead is something like "LUFFY, THE WORLD GOVERNMENT EXECUTED MY FATHER AND AKAINU WANTS TO IMPOSE MARTIAL LAW ON EVERY KINGDOM. PLEASE LET ME JOIN THIS TIME SO WE CAN STOP THEM AND SAVE ALABASTA!", which does not feel synergetic or interesting at all compared to all of the other Straw Hat joining declarations.
And then there is the fact that we have to come to terms with finally acknowledging that Vivi's flashback counts as a full-fledged Straw Hat backstory. I bet you forgot she even had a flashback before reading this, didn't you? Don't you remember that traumatic life changing backstory when she played with Kohza as a kid, and I think some thieves tried to kidnap one of them for a ransom, and they played in the clock tower where Crocodile hid his bomb years later…? Yeah, it is very telling how forgettable that flashback was when everybody can still remember Zoro and Usopp's backstories that lasted only one-two pages long and felt so much more mundane everybody else's tragedies. That is easily the worst flashback in the entire series. Try to disagree, I dare you. Now that I think about, Vivi might have the one backstory I can actually believe is less interesting than Carrot's last minute flashback snippets right after Pedro's suicide bombing.
Also, as nostalgic as Vivi is, I still want a new design for the final crewmate that has neither a case of Namiface or light blue hair like Franky. Hell, I would say the latter creative decision on Oda's part outright confirms Vivi is never coming back as a permanent crewmate in the same way that he wanted Bon Clay's voice actor to voice Franky instead of a having a new voice actor like the other Straw Hats if he were to ever actually join one day. Although I must admit that the theory where Vivi's crew member number being 5.5 in the SBS actually means 11 if Carue also counts at 5.5 is cute, I like those kinds of clever hints.
Furthermore... I know I said this already, but again, those peacock slashers are so damn lame. I will never think they look cool even if they somehow work better than a Stormtrooper's blaster aim now. I don't know why, but they bother me when I compare them with everybody else's offensive specialties. I'd rather have a third swordsman than see those things again lol.
She can come back as an ally in the Final War. Heck, maybe she could even lead the entirety of the Grand Fleet to cement her honorary Straw Hat status (although that gets in the way of Usopp finally leading 8000 followers at least once). But try to imagine all eleven Straw Hat members approaching Raftel, redeclaring their dreams on a barrel just like the first five Straw Hats did as they entered the Grand Line, and Vivi blurts out either "I'm doing this for my kingdom!" or "I want to be the best princess ever!" It's just... awkward.
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