Well, perhaps in this fanfiction, Luffy time - travels to the time when Shanks lost his arm. Luffy could use his current strength and knowledge to prevent that from happening. This would change the course of Shanks' life and also have a big impact on Luffy's own growth. Or, he could travel to a time when the World Government was first established. Luffy might try to understand the roots of the corruption there and start his fight against it even earlier.
One idea could be Luffy travels back in time to the era when Gol D. Roger was still alive. He could learn from Roger about the true nature of the One Piece and gain new insights to use in his own journey. Another could be that he goes back to a time when the Straw Hat crew was just forming and he gets to relive those early days with new knowledge, making different choices to strengthen the crew faster. Maybe he travels to a future time to see the consequences of his actions and then comes back to change things for the better.
One interesting plot idea could be that Luffy and Law travel back in time to the era of the great pirates before Gol D. Roger. They could meet young versions of legendary pirates and learn secrets about the One Piece that have been long forgotten. For example, they might encounter Whitebeard as a young and ambitious pirate, and through their interactions, gain insights into the true nature of the power of the Devil Fruits. Another idea is that they travel to a future time where the world is in chaos due to a new and powerful enemy. They have to adapt to the advanced technology and strange new powers in that era to find a way to save the world and bring back peace.
One idea could be that they travel back to the time when the Straw Hat Pirates first formed. Luffy could use his knowledge of the future to make the crew even stronger from the start. For example, he might know about certain enemies in advance and train the crew to better face them. Nami could also use her knowledge to help with things like finding better treasure routes earlier.
One idea could be that Luffy travels back to the time when Gol D. Roger was still alive. He could meet the Pirate King and learn some of his secrets. Maybe Roger gives him a hint about the One Piece that changes Luffy's understanding of his journey.
One idea could be Luffy learning new and extremely powerful Haki techniques during the time skip that no one has ever seen before. This would make him even more of a force to be reckoned with when he returns to his crew.
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.
A possible plot in the fanfiction could be that they time travel to a point where the Straw Hat crew has disbanded in the future due to some great tragedy. Luffy and Nami then have to figure out what went wrong and change the course of history to reunite the crew. Or perhaps they go back to a time when Nami was first forced to work for Arlong. Luffy, with his knowledge of the future, could find a way to free Nami much earlier and change the course of her life and the crew's development.
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.
Another concept is Luffy traveling to the future. He could see how his actions have shaped the world of pirates. He might find out that his name has become a legend and inspired many new pirates, which would be really cool for him.
One idea could be that Tinkerbell discovers a hidden map in Pixie Hollow that leads to a long - lost pirate treasure. Along the way, she teams up with some of the pirate fairies and they face various challenges like tricky sea creatures and bad - weather magic. Another plot could be that a new pirate fairy arrives in Pixie Hollow with a mysterious power that Tinkerbell has to figure out whether it's good or bad for their world. Maybe this new fairy is being chased by an old enemy from the pirate world and Tinkerbell gets caught in the middle.
One idea could be a pirate crew made up of Hogwarts students who go on a quest to find a magical treasure hidden by an ancient wizard - pirate. They'd use their magic in unique ways on the high seas. Another could be a story where Harry discovers a long - lost relative who was a pirate and inherits a magical pirate ship. The ship has its own secrets and powers.