One Piece has often demonstrated that any element of fiction may be included into the storyline. The series essentially covers everything, from Giants to Dwarves, Mermaids to Ninjas. There are a few predictions in it as well that will be significant in the Final Saga. A startling prophecy in the "Fishman Island" arc claims that Luffy would be the one to destroy Fish-Man Island; Madam Shyarly sees him standing between the flames with a serious expression on his face. Nobody knows exactly when this will occur, but Luffy's involvement in the prophecy says it probably won't be too long from now.
Even when she foretold Whitebeard's demise, Madam Shyarly's prophesies were always accurate. She smashed the crystal ball that let her to see visions after realizing Luffy wasn't a bad person after their struggle with Hordy Jones was done. Madam Shyarly is revered by the residents of Fish-Man Island for her skills. The prophecy of their island being destroyed could have a deeper and more uplifting significance given their particular situation and Luffy's character.
The Residents of Fish-Man Island's Greatest Desire
The residents of Fish-Man Island yearn to live in the same climate as the world of One Piece. The Sunlight Tree Eve's roots collect sunlight on the ocean's surface and transmit it to Fish-Man Island through the tree's roots, so the sunlight they experience deep beneath the ocean's surface is artificial.
However, the Fishmen have been living underwater because of human prejudice. Even beneath the ocean, they often face danger unless they are associated with powerful pirates like Whitebeard or Big Mom. The late queen Otohime did everything she could to improve the relationship between Fishmen and Humans, and came very close to achieving her dream of harmony. Sadly, her tragic death and Hordy Jones' schemes wasted all her efforts.
Why It Seems Like Luffy Might Destroy Fish-Man Island
Given Luffy's character in One Piece, it seems impossible for him to bring ruin to an island, let alone his own homeland. Luffy is a crucial ally to Fish-Man Island now that he is a Yonko and has Jinbe, the most powerful and esteemed Fishman, in his crew. As a result, the prophecy of Madam Shyarly only makes sense if Luffy is somehow compelled to destroy the island by his foes.
The Fishmen will be able to live freely on the surface world and fulfill their long-held desire once the island is gone. They already face less prejudice than in the past, and perhaps this trend will continue. So, it is not impossible to think that Luffy will establish a safe haven for the Fishmen under the actual sun once he has achieved all of his objectives.