Character development is key. Fanfictions explore how Ace and Luffy grow under Garp's influence. Maybe Luffy becomes more disciplined in some stories, or Ace learns to be more open - hearted, all thanks to their interactions with Garp and each other.
Luffy and Ace's interaction is always key. In fanfiction, it could be that Ace reveals a secret to Luffy right before the final battle that gives Luffy newfound strength. Another great interaction could be between Luffy and Garp. Garp might be conflicted between his loyalty to the Marines and his love for his grandsons, and this inner turmoil could lead to some really emotional and tense moments when he interacts with Luffy at Marineford.
One possible plot could be that Luffy gets into a really tough situation during his adventures, perhaps surrounded by powerful enemies on an unknown island. In desperation, he calls out to Garp for help. Garp, despite his tough exterior, races to his grandson's aid. Along the way, he might reminisce about Luffy's growth and how far he has come.
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 could be that Gear 5 Luffy arrives at Marineford during the big battle. He might use his newfound powers to turn the tides of the fight. For example, he could use his rubbery abilities in even more extreme ways, like stretching to deflect cannonballs aimed at his allies and then bouncing them back at the enemy ships. His presence would shock both the Marines and the pirates. He could also engage in a one - on - one fight with some of the top Marine officers in a more comical yet powerful way, as Gear 5 gives him a more carefree and powerful fighting style.
Perhaps in this fanfiction, Ace, Luffy and Sabo will make Marineford a place where pirates and marines can live in harmony. They could set up some rules that both sides can abide by.
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.
In addition to Luffy and Garp, other characters could also play important roles. For example, some of Luffy's crewmates like Zoro or Nami might be present when Luffy calls Garp. They could add their own perspectives and reactions. Also, there could be some marines around Garp when he receives the call, and their reactions to Garp's decision to go to Luffy could also be part of the story.
In this fanfiction, Ace, Luffy and Sabo could have a future in Marineford that is full of hope. Maybe they work together to change the perception of pirates in that place. Ace could use his fire power to protect the weak, Luffy with his unwavering will continues to inspire others, and Sabo with his intelligence devises plans to make Marineford a more inclusive place for both pirates and marines.
A possible plot is that Luffy discovers a time - traveling device at Marineford. He then uses it to go back in time briefly to prevent some of the worst events from happening, like Ace's capture. Or perhaps Luffy has a dream where he is given advice by a long - dead pirate on how to win the battle at Marineford. This advice then helps him make better strategic decisions during the real battle. He could also meet a mysterious character at Marineford who has the power to heal all the wounded, which changes the whole course of the battle.