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 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.
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 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.
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.
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.
If ASL time - travels, Luffy's relationship with his brothers would be strengthened in new ways. They would have to rely on each other more as they face the unknowns of the past. For example, Ace might see a younger Luffy and be even more protective.
Maybe she travels to a time during a great war. She could end up changing the course of the war by providing crucial information or by using her skills in a way that no one expected. For instance, if she found a way to counter an enemy's powerful jutsu that was thought to be unbeatable at that time. This would not only affect the outcome of the war but also change the future of the ninja world she came from.
One idea could be Madara traveling back to the era of the Sage of Six Paths. He could try to change the very foundation of ninjutsu, which would create a whole new timeline. Maybe he would attempt to gain the power of the Sage for himself much earlier in history.
Well, perhaps a time - traveling device accidentally sends Judy Hopps back to a time when prey and predator relations were extremely tense. She has to use her wits and modern - day knowledge to prevent a major conflict that could have changed Zootopia's future for the worse. Or, a time - traveling fanfiction could focus on Nick Wilde going back to his own past, where he can make different choices that would impact his relationship with Judy and his standing in Zootopia society in the present.
One idea could be a character from the present in BO2 traveling back to a crucial moment in the game's history to change the outcome of a major battle. This would create a whole new timeline and a lot of interesting interactions with past characters.