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.
If Luffy goes back far enough to change the events at Water 7, perhaps Usopp would not leave the crew. This would prevent the internal conflict that the crew faced at that time. The crew would be more united from the start, and they might be able to face Enies Lobby even more effectively. Their bond would be stronger, and it could lead to different power - up scenarios for the crew members as they would be more focused on growth together.
Another idea is that they time travel to a random point in the past, like when Gol D. Roger was still alive. Luffy would be excited to meet the Pirate King. Roger might see something special in Luffy and give him some cryptic advice. Robin could ask Roger about the secrets he knew about the world's history. This encounter could have a huge impact on Luffy's journey and how he views his role as a pirate.
In such a fanfiction, Luffy could potentially prevent some of the tragedies that occurred in the original story. For example, he might be able to save Ace from his untimely death. He could also use his knowledge of future events to form stronger alliances earlier and gain more powerful allies. This could lead to a much more powerful Straw Hat crew and a different power balance in the One Piece world.
In some One Piece Luffy Gear 5 fanfiction, there could be the idea of Luffy sharing his Gear 5 power with his crewmates temporarily. This would not only make for some cool team - up attacks but also show the deep bond within the Straw Hat crew. Also, a fanfiction might explore how Luffy's Gear 5 affects his relationship with other pirates or the World Government. Maybe it makes him an even bigger target or maybe it causes some pirates to want to ally with him out of fear or respect.
One idea could be a story where Luffy, Nami and Robin get stranded on a mysterious island. Luffy is eager to explore, Nami is worried about resources and Robin is interested in the island's history. They work together, with Luffy's leadership, Nami's navigation skills and Robin's knowledge to survive and find a way off the island.
One possible idea for a fanfic where Luffy is a girl could be that she has a different approach to leadership. Instead of the brash and straightforward style, she might use her charm and a more empathetic side to bring the crew together. For example, she could be more in tune with the emotional needs of her crewmates like Nami and Robin, and this could lead to deeper and more meaningful relationships within the crew.
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.
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 in a fanfiction, Bonney could be a stowaway on the Thousand Sunny. Luffy discovers her and instead of being angry, he welcomes her as part of the crew. Then they go on adventures together, with Bonney learning about Luffy's dream and starting to believe in it too.
One idea could be Luffy accidentally eating a Devil Fruit that gives him the ability to create lemon - scented mirages. He uses this to fool his enemies during battles. Another could be a story where Luffy discovers a hidden island full of lemon - based treasures that are said to have special powers. And perhaps a plot where Luffy teams up with a lemon - loving character who has a unique fighting style that involves using lemons as weapons.