Maybe Luffy goes back to train harder and acquire new powers earlier. That could lead to some exciting battles and unexpected plot twists.
One possibility could be Luffy changing key events to prevent the losses of his dear friends. This would add a lot of drama and emotional depth to the story.
One possible scenario could be that Luffy sacrifices himself to save his crew during a major battle against an extremely powerful enemy. He might use all his strength and energy to protect them, and in the end, it's too much for his body to handle.
In Luffy fanfiction, a common scenario of jealous Law is when it comes to making alliances. Luffy is so good at making friends and getting others on his side. Law may feel jealous that Luffy can do it so effortlessly while he has to work harder. Also, when they are on an island exploring and Luffy discovers some amazing treasure or secret first, Law might get jealous of Luffy's luck.
One possibility could be that Luffy goes back in time and decides to prevent the death of Ace. He might use his knowledge of future events to stop the Marines from getting to Ace in the first place. This would change the entire course of the story, as Ace's death was a major turning point for Luffy and the Straw Hat crew.
In some 'Luffy marries Nami' fanfiction, there could be a scenario where they get married on the Thousand Sunny. All the crew members are present, and Sanji prepares a grand feast for the occasion. Luffy is grinning widely, and Nami is blushing as they exchange vows.
One interesting scenario could be that they team up against a powerful new enemy. They combine their unique skills, Luffy's rubber abilities and Sabo's fire - based attacks, to create a formidable force.
One interesting scenario could be that while they are napping, they start having the same dream about their next big adventure on the Grand Line. In the dream, they are facing a powerful enemy but they work together seamlessly, just as they do in real life.
Luffy could change the past in several ways. Firstly, he could use his devil fruit powers in a more strategic way. Since he has the experience of future battles, he knows his opponents' weaknesses better. So, when facing the enemies who were involved in Ace's capture, he can defeat them more easily. Secondly, he could try to influence Ace's relationships. Maybe he can make Ace form stronger alliances with other pirate crews, which would make it more difficult for the Navy to target him. Also, Luffy could try to expose the Navy's secret plans earlier, which would change the power dynamics in the One Piece world and ultimately save Ace.
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.
Well, there could be all sorts of changes. Maybe he'd get a chance to fix past mistakes or prevent certain events from happening.
If Kylo Ren went back in time in fanfiction, he might try to change the events that led to his fall to the dark side. Maybe he would go back to a time when he was still Ben Solo and prevent himself from being influenced by Snoke. He could use his knowledge of the future to avoid the mistakes he made.