The story of One Piece looks at the world from the perspective of Luffy who wants to become the Pirate King, so what if the protagonist is a marine? The outline of the story remains unchanged, only the protagonist of the book is added to promote the development of the story.
Protagonist: Battle, the grandson of the Warring States Period, the child who was picked up, was taken to Windmill Island when he was a child, and met Luffy Ace Sabo. When they were bullied, they became friends, and then left. He wanted to become a marshal, and his master was Akainu.
Justice is everything! Let's see what happens if we develop from the naval direction.