It could be a time - traveling pirate who has his own evil plans for the past. He might be trying to change the past to gain ultimate power in the future. So he becomes an obstacle for Luffy who wants to either preserve the past or make positive changes.
Well, he could go back to the time when his crew first formed. Luffy might use his knowledge of the future to make their early adventures easier, like knowing exactly where the treasure is on certain islands. Or he could travel back to the time of the great pirate wars and try to stop some of the bloodshed. This would change the power dynamics in the world and create a whole new set of challenges and allies for him.
In such a fanfiction, Luffy could potentially prevent some of the tragedies that occurred in the original story. For example, he might be able to save Ace from his untimely death. He could also use his knowledge of future events to form stronger alliances earlier and gain more powerful allies. This could lead to a much more powerful Straw Hat crew and a different power balance in the One Piece world.
She could meet the founders of Hogwarts. Imagine Hermione having a conversation with Rowena Ravenclaw about the importance of knowledge and wisdom. She might also meet Godric Gryffindor and learn about the true meaning of courage from him. Another person she could meet is Salazar Slytherin. It would be very interesting to see how she would interact with him, given his views on blood purity.
His personality might be more serious in some ways. Since he has the knowledge of the future, he might be more cautious about certain actions. But at the same time, he'd still be the fun - loving Luffy. For example, he'd still love meat, but he might be more strategic when approaching battles or dealing with powerful enemies, like Blackbeard.
I can recommend you a novel that travels back to ancient times. It's called " The Peerless Beauty in Ancient Times." This novel was about a modern girl who accidentally traveled to ancient times and became a beautiful woman. She experienced all kinds of tests and challenges in ancient life and finally obtained her own happiness and achievements. The plot of the novel is full of twists and turns, the characters are vivid, and the language is beautiful. It is the best choice for you to read.
One possibility could be Luffy changing key events to prevent the losses of his dear friends. This would add a lot of drama and emotional depth to the story.
Luffy could change the past in several ways. Firstly, he could use his devil fruit powers in a more strategic way. Since he has the experience of future battles, he knows his opponents' weaknesses better. So, when facing the enemies who were involved in Ace's capture, he can defeat them more easily. Secondly, he could try to influence Ace's relationships. Maybe he can make Ace form stronger alliances with other pirate crews, which would make it more difficult for the Navy to target him. Also, Luffy could try to expose the Navy's secret plans earlier, which would change the power dynamics in the One Piece world and ultimately save Ace.
In some cases, the antagonist could be Esme herself, or rather a version of her from the past. The past Esme might be more naive or self - centered and not understand the implications of Esme's actions in the past. The present Esme is trying to make positive changes, but the past version of herself keeps getting in the way, either through her own actions or by influencing the people around her in a negative way.