Often, a common plot point is the clash of their different worlds. Luffy, a pirate, and Sandersonia, from a different background, have to bridge that gap. For example, in some fanfics, Sandersonia is initially distrustful of Luffy's motives.
One great luffy x sandersonia fanfic is 'The Unlikely Bond'. It beautifully explores their relationship in a world full of adventures. The story shows how Luffy's carefree nature and Sandersonia's strong personality interact and grow closer over time.
One common theme could be adventure. Given the nature of the source material (assuming it has elements of action or exploration), fanfics might explore new adventures for the characters.
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.
Some fandom - specific forums or communities might also host boa sandersonia fanfic. These are often more niche but can have some really creative and unique works. You could try joining relevant groups on social media platforms and asking for recommendations from other fans.
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 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.
It could be a time - traveling pirate who has his own evil plans for the past. He might be trying to change the past to gain ultimate power in the future. So he becomes an obstacle for Luffy who wants to either preserve the past or make positive changes.
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.