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 might start challenging the Fairy Tail wizards to friendly battles. Since Luffy is all about adventure and getting stronger, he'd see the powerful mages in Fairy Tail as great opponents to test his skills against.
One interesting aspect could be that Luke goes back to a time when the Sith were just emerging. He could try to root out the Sith threat before it became as powerful as it did. He might have to be very careful not to disrupt the timeline too much, but his Jedi wisdom could be a great asset in this time - traveling adventure.
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.
I think Luffy would befriend Lucy first. Lucy is kind - hearted and she has a similar sense of adventure as Luffy. She also has a collection of celestial spirits which would be really interesting for Luffy. He might see her as a valuable ally and friend right away. And with Lucy being one of the more approachable members in Fairy Tail, it's likely that they'd start a conversation and become friends quickly.
Luke could also go back to meet his father, Anakin, when he was still a young boy. He could be like a mentor to him, teaching him about the Force in a different way. This might prevent Anakin from ever turning to the dark side, which would rewrite the entire Star Wars story. He could introduce Anakin to different ways of using the Force that are more positive and less focused on power and control.
One possible plot twist could be that when Luffy goes back, he discovers that Ace's fate is more intertwined with the World Government's secrets than he thought. Maybe Ace was targeted not just because of his bloodline but because he knew something dangerous. Another twist could be that in the past, Luffy has to team up with unexpected allies, like some of the former enemies of the Straw Hats, to save Ace. And perhaps, his time - traveling disrupts the timeline in a way that creates new threats not only to Ace but to the entire world of One Piece.
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.
The Grand Secretary of the Supreme Court was not exactly the same as the Grand Secretary of the Supreme Court. The Head of the Supreme Court was the chief of the Supreme Court, equivalent to the Supreme Court in modern times. He was responsible for handling criminal cases. The Deputy Official of the Supreme Court was the Deputy Official of the Supreme Court. He assisted the Supreme Court Official in handling cases and was responsible for some paperwork, records, and prison work. Thus, although both of them held positions in the Supreme Court, their ranks and responsibilities were different.
Nami would be very practical. She'd likely start by making sure she has all the necessary supplies for a healthy pregnancy. She might ask Chopper, the crew's doctor, for advice on nutrition and health. And she'd probably try to find a way to continue her role as the navigator, perhaps creating new maps that include safe places for when the baby is due.