Well, in this fanfiction, Luffy could interact with the Marines in a more diplomatic way. Since he has the advantage of time travel, he might understand the reasons behind the Marines' actions better. So, he could try to negotiate or make deals with some of the more reasonable Marines. He could offer to help them with other threats in the One Piece world in exchange for them leaving his friends alone at Marineford.
One possible plot development could be that his death shatters the Straw Hat crew. They might disband in grief, each going their own way. Zoro could become consumed with revenge, vowing to take on the Marines who were responsible.
Nami would probably be heartbroken. She has been with Luffy through so much and his death would leave her feeling lost and alone. She might even lose her will to continue sailing at first.
Well, in the fanfiction, Gear 4 Luffy might have had some new and unexpected interactions. Maybe he could have teamed up with some other pirates in a different way. Since he was in such a powerful form, he could have inspired more of the pirates to fight back more bravely against the Marines. And his presence in Gear 4 could have also affected the admirals' strategies, making them more cautious or forcing them to use their more powerful techniques earlier.
It would likely make the storyline more exciting. Gear 4 Luffy is extremely powerful, so his presence in Marineford would change the power dynamics. He could defeat more enemies quickly.
One possible plot twist could be that Luffy's time travel alters the fates of some of the characters who died at Marineford. For example, he might be able to save Ace. Another could be that his presence in the past changes the future relationships between the pirates and the Marines in unexpected ways. Maybe some pirates who were once enemies now become allies because of Luffy's interference.
Yes. The conflicts between Sanji and Luffy in the manga often stem from complex circumstances and character developments that drive the narrative forward.