One possible plot twist could be that Luffy and Robin travel back in time to the Void Century. This would open up a whole new world of exploration as they discover the true history of the world. They might encounter the Ancient Kingdom and learn the secrets that have been lost for so long. It could also put them in a dangerous position as they might be hunted by the World Government who wants to keep that history buried.
Perhaps a plot twist could be that when they time travel, they switch bodies. Luffy would be in Robin's body and vice versa. This would create a lot of comical situations at first, but also pose serious challenges. They would have to adapt to each other's abilities and ways of thinking. Luffy in Robin's body would have to figure out how to use her Devil Fruit powers properly, and Robin in Luffy's body would have to deal with his stretchy powers while also leading the crew in a different way.
A really interesting plot twist could be that in the time - travel, Luffy and Zoro switch bodies. This would lead to a lot of comical situations as they try to adapt to each other's abilities. Zoro in Luffy's body would have to deal with the stretchy powers, and Luffy in Zoro's body would have to handle the three - sword style. It would also create some unique battle strategies as they fight enemies.
Another idea is that they time - travel in a loop. Every time they go back in time and change something, it creates a new future, but then they get sent back again because the change wasn't enough to break the loop. They keep meeting different versions of their friends and enemies in different time periods. It's a never - ending cycle until they figure out the key to break the loop and return to their normal time with a better understanding of how their actions can shape the future.
One possible plot could be that Luffy and Robin get sent back in time to the era when the Ancient Weapons were first created. They might have to figure out how to stop a powerful force from misusing the weapons. Luffy would use his determination and unique abilities, while Robin's knowledge of history and ability to read Poneglyphs would be crucial in understanding the situation and finding solutions.
One possible plot twist could be that Narukaka travels back in time and accidentally changes a small event that has huge, unforeseen consequences in the future. For example, preventing a minor meeting between two people that was supposed to lead to a great discovery.
One possible plot twist could be Sandor traveling back to his childhood and preventing the incident that scarred him. This would completely change his character arc and his relationship with his brother. He might become a more confident and less bitter person.
One possible plot twist could be that Naruto and Jiraiya travel back to a time when the Uchiha clan was at its peak. This would put Naruto in a position to potentially prevent the Uchiha massacre. He could use his knowledge of future events and his skills to change the course of history for the better. Jiraiya, on the other hand, could discover secrets about the Uchiha that were lost over time.
One possible plot could be that after many adventures and saving the world together, Luffy realizes his true feelings for Robin. They then decide to get married on their ship, with the whole crew celebrating.
One possible plot twist could be that Whitebeard actually kidnaps Luffy to protect him from a greater danger that Luffy is unaware of. Maybe there's a powerful enemy lurking in the shadows that Whitebeard knows will target Luffy soon, so he takes this extreme measure to keep him safe in his own territory.
One of the most interesting plot twists could be when a character from the original 'The Godfather' story travels back in time and accidentally changes a major event that alters the whole family's future. For example, if Michael Corleone went back and prevented his father from being shot, it would completely change the power dynamics and the family's path in the mafia world.
In Robin and Luffy time travel fanfiction, a frequently seen theme is the exploration of history. Since they are time traveling, they get to see the One Piece world's past up close, which includes things like the origin of devil fruits or the history of certain kingdoms. Another theme is the strengthening of their bond. Through facing different challenges in different time periods together, their friendship and trust in each other grow even deeper. And of course, the theme of destiny also often appears. They might question if their time travel was fated and what role they are supposed to play in the different timelines.