The other characters' development would be completely derailed. For example, Chopper, who looked up to Luffy as a source of inspiration, would probably lose his confidence. Robin, who had found a new family in the Straw Hats with Luffy as the core, might isolate herself again. Without Luffy, the bonds that held the crew together would start to crumble, and their individual character growth would be stunted.
The impact would be huge. First, the Straw Hat crew would be devastated. They would likely lose their motivation and purpose. Zoro might go on a vengeful spree. Also, the whole world of pirates would be shaken. Luffy was seen as a rising star, a symbol of hope for many. His death could lead to a power vacuum that other pirates would fight to fill.
Well, Neville would also be affected. Harry was an inspiration to Neville, and without him, Neville might struggle more with his confidence. However, it could also be a turning point for Neville, where he has to find his own inner strength without Harry's example. Ginny would be heartbroken. She had a special connection with Harry. Her character might become more reserved and less outgoing as she deals with the loss. For the Hogwarts professors, they would have to deal with the loss of a student in a different way. McGonagall, for example, would be deeply saddened and might become more protective of the remaining students. Snape would have a different emotional response as well, as his complex relationship with Harry was a big part of his character development.
In this type of fanfiction, one possible plot development could be that Luffy sacrifices himself to save Nami from a powerful and unbeatable enemy. Maybe an enemy with a devil fruit power that can only be countered by Luffy's unique abilities, and in the process of protecting Nami, he exhausts all his strength and ultimately dies.
In this fanfiction, the two central characters are Ace and Luffy. Ace, being the older brother, is likely to be the more protective and guiding one. Luffy, on the other hand, is full of energy and curiosity, always ready for an adventure. Their different personalities will surely lead to many interesting events in the story.
Well, 'Luffy and Ace: Unbreakable Ties' is a fanfiction that I really like. It focuses on their childhood memories and how those shaped their relationship as they grew up. There's also 'Ace and Luffy's Great Escape' which is full of action. It shows their cleverness in getting out of difficult situations. 'Luffy and Ace: The True Friendship' is a must - read too. It simply and effectively shows the depth of their friendship.
Obviously, Ace and Luffy's crew which includes Luffy himself, Zoro, Nami, Usopp, Sanji and others are the main characters.
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.
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.
One great 'Ace Sabo Luffy Fanfiction' is 'The Brothers' Bond'. It beautifully explores their relationship, showing how they support each other through various challenges. Another one is 'Sabo's Return'. It focuses on Sabo coming back into Luffy and Ace's lives and how they all readjust. There's also 'Ace and Luffy's Adventure', which takes the three on a fictional journey filled with action and heartwarming moments.
Well, one characteristic could be the focus on a more 'romantic' or 'intimate' relationship between Luffy and Ace, which is often not in line with the original platonic nature of their bond in the source material. However, this is a very controversial aspect as it goes against the typical portrayal of their brotherly relationship.