Gol D Roger was the Pirate King. He amassed a great fortune and reached the final island, Laugh Tale. His execution led to the Great Pirate Era as he revealed the existence of One Piece before his death. His crew, the Roger Pirates, were among the most powerful and feared at that time. Roger's journey was filled with epic battles, exploration, and a quest for the ultimate treasure.
I don't have enough context to determine exactly who 'garp' is. It could be a newly created character by the fanfic writer, or it might be a character from a relatively unknown story that is being incorporated into the 'ace fanfic'.
Roger's navigational skills and knowledge of the Grand Line could be paired with Naruto's ability to sense danger and his quick reflexes. When they are on a ship together, Naruto can sense approaching threats while Roger charts the best course, and they use their combined skills to fend off enemies.
It's a fictional concept. Harry Potter and Gol D. Roger are from two completely different fictional universes. Harry Potter is from the wizarding world created by J.K. Rowling, while Gol D. Roger is from the One Piece universe. Combining them in such a fanfiction way is a creative and fictional idea that doesn't exist in the official storylines of either franchise.
In this fanfiction, perhaps Luffy would get some valuable advice from Gol D Roger. Roger could share his experiences about the One Piece and the Grand Line, which might give Luffy new insights on his journey to become the Pirate King.
One idea could be a time - travel plot where Naruto somehow gets sent to the One Piece world during Gol D. Roger's era. He could learn from Roger and use his ninja skills in the pirate world.
The discovery of the existence of One Piece was a crucial part of his story. Also, the formation of his powerful crew, the Roger Pirates, was a significant event. His journey across the Grand Line and the New World, facing various challenges, is also key.
Obviously, the god child OC is one of the main characters. And there might be characters who are either protecting or trying to harm the god child because of their relation to Gol D. Roger.