Well, he could time - travel to a point where the Straw Hats first formed but he's the only one who remembers the future. This would lead to some really funny and interesting situations. He might try to prevent some of the early hardships they faced, like the fight with Buggy. Or perhaps he travels to a time during the Void Century and discovers secrets that could change the course of the current One Piece world. He then has to figure out how to bring this knowledge back to his own time without causing chaos.
One idea could be a story where Sanji and Usopp get stranded on a strange island together. Sanji has to rely on Usopp's creativity to find food sources that are not immediately obvious, while Usopp depends on Sanji's combat skills when they encounter dangerous creatures on the island. This would force them to bond and understand each other better.
Well, a really cool plot could be about a time - traveling device falling into the hands of a villain. This villain then goes back in time to stop Luffy from ever starting his journey. The Straw Hats would then have to find a way to also travel back and stop the villain. Or, a character could travel back in time and accidentally change something small that has huge ripple effects on the present One Piece world, like changing the location where a Devil Fruit was found, which could change who got what power.
One interesting plot idea could be a character from the present-day One Piece world traveling back to the time of the Void Century. They could discover the true history of the world and the origin of the Ancient Weapons, which would completely change the power dynamics in the present. Another idea is a character from the past traveling to the future and being shocked by how the world has developed, with new islands, new Devil Fruits, and different pirate crews ruling the seas.
One interesting plot could be that Ace, Sabo, and Luffy travel back in time to when Gol D. Roger was still alive. They could try to prevent certain events that led to the current state of the One Piece world. Maybe they want to stop the tragedies that befell their families in the past.
One idea could be Ace traveling back to the time when Gol D. Roger was still alive. He could witness the events that led to Roger's surrender and execution, and perhaps even interact with the Pirate King. This would give him new insights into the world of piracy and his own place in it.
In the past, Usopp's abilities like his slingshot skills might be seen as more primitive compared to the advanced weaponry in his own time. But his creativity could give him an edge. For example, in a time when there were less advanced long - range weapons, his slingshot could be a very unique and effective tool. In the future, if he travels there, his abilities might seem obsolete at first. However, he could adapt by using his knowledge of old - fashioned combat and combine it with the new technology he finds.
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.
Perhaps a magical curse befalls the crew that causes several female characters in the One Piece world to be irresistibly drawn to the male members of the Straw Hats. This could lead to all sorts of chaos as the crew tries to deal with these new, unexpected relationships while still pursuing their main goals.
A plot could involve a supernova teaming up with an unexpected character, like a former villain who has a change of heart. They could go on a quest to find a long - lost treasure that has a special connection to the supernova's past. This would not only create an interesting dynamic but also explore the backstory of the supernova character more deeply.
Well, a plot could involve a vampire character who was once a regular pirate but got turned into a vampire. This vampire pirate then has to deal with his new cravings while still trying to achieve his original goals in the One Piece world, like finding the One Piece. And maybe there's a group of vampire hunters in the One Piece world who are after him.