When the Straw Hat Pirates visit Fish-Man Island again, I suspect the Island’s deeper relevance to the Void Century will be covered as the story threads of Poseidon, the “Ship of Promise” Noah, Wano and Luffy are weaved together. Obviously when that time comes, the Straw Hat Pirates are going to need someone who is both aware of Joy Boy and familiar with the Mythical Zoan Dragon Devil Fruit. Thankfully, Oda has crafted a character that fills such a purpose in the Wano arc i.e. Yamato.
By nature of Joy Boy being intimately connected to Fish-Man Island, it can be extrapolated that at one point Fish-Man Island were close with the Ancient Civilisation and supportive of their intentions. The fact that Fish-Man Island housed several Poneglyphs including a Road Poneglyph before it was relocated (by Shanks?) speaks to level of connection between the two races and their role in the path toward the “Dawn”.
As mentioned above, the real and artificial Mythical Zoan Dragon Devil Fruits Kaido and Momonosuke respectively possess will hold relevance when the story of Fish-Man Island is brought into focus again. Considering the reverence attached to the imagery of a Dragon which the Ryugu Palace was themed around, it implies an affinity between the statue and the Devil Fruit form. Aside from the size, colour and Kaido’s horns, the appearance of the Dragon forms between the three match completely. With such similitude, designs intentional by Oda, there certainly exist a connection between the Ryugu Palace Dragon statue and the Uo Uo no Mi, Model: Seiryu. And such a relation WILL eventually be explored.
As for the reason why Fish-Man Island attaches such veneration to the imagery of the Dragon, I suspect it may have to do with the role the Dragon played in their past. It may be possible that before Devil Fruit’s became a thing, Joy Boy possessed the ability to transform into a Dragon (a type of Fish that was able to swim in the ocean) which was eventually inherited by a Fruit – the first Devil Fruit? Or Joy Boy could have associated himself with the creature that is now represented by the theme of the Ryugu Palace. Joy Boy was likely able to converse with life forms (similar to Momonosuke/Oden/Roger/Luffy/Shirahoshi) and through such an ability bonded with the [Fish] Dragon. That connection may be what is being mirrored currently in Luffy and Momonosuke’s (Dragon form) battle against Kaido.
I also wouldn’t be surprised if by some twist of fate, the story also connects the Kozuki Clan with Fish-Man Island – the Kozuki stonemason being the ones to craft the [Fish] Dragon statue. The presence of Poneglyphs being on Fish-Man Island evidences the suspicion that the Kozuki Clan at one point was hosted by Fish-Man Island. Joy Boy may very well be a member of the Kozuki Clan. When the story is ready to delve into such details, having someone (Yamato) connected to the Mythical Zoan Dragon Devil Fruit (Kaido) and Wano will assist tremendously.
At present, Luffy has no reason to destroy Fish-Man Island. The precursor to such an act may very well be induced by the knowledge and context Yamato provides.
Yamato isn’t just a character to serve as exposition but rather a character intentionally created by Oda to assist in moving the story of Joy Boy and the Ancient Civilisation forward when such a plot so far has been largely isolated from the Straw Hat Pirates adventure. With Oda’s sights set on the conclusion of One Piece, it is about time the Straw Hat Pirates align their path with that of Joy Boy’s and Yamato, as a nakama to Luffy, will assist in achieving such an end.