The World Government from an earlier era could also be the main antagonist. They might be more powerful and oppressive in the past. Luffy, with his modern - day knowledge and values, would clash with them as he tries to do things like free slaves earlier or prevent certain unjust laws from being made.
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.
It could still be Orochimaru. Even if Sasuke travels back in time, Orochimaru's thirst for power and knowledge wouldn't change. He would still be plotting to gain power, and Sasuke, with his knowledge of Orochimaru's future actions, would be in a position to stop him. Orochimaru's experiments and his desire to take over the bodies of others make him a formidable antagonist.
Danzo could still be a major antagonist. Even if Naruto travels back in time, Danzo's thirst for power and his underhanded methods would still pose a threat. Naruto would have to find a way to deal with him without causing too much chaos in the village.
It could be a time - traveling version of Zeref. He might be trying to stop Natsu from changing things in the past that could potentially lead to his own downfall in the future.
Aizen could still be the main antagonist. Even if Ichigo travels back in time, Aizen's long - term plans might be already in motion. Ichigo would have to find a way to expose and stop Aizen earlier than in the original timeline.
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.
Well, there could be all sorts of changes. Maybe he'd get a chance to fix past mistakes or prevent certain events from happening.
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.
An ancient sea devil who was sealed away in the past but is awakened by their time - traveling. This devil could have the power to manipulate time itself and tries to stop Luffy and Nami from returning to their own time or achieving their goals.
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.
It could be a time - traveling version of Blackbeard. Maybe he also discovers time travel and uses it to his advantage to stop Luffy at every turn. He could go back in time and try to prevent Luffy from getting important allies or Devil Fruits.