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.
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.
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.
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.
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.
One interesting Ace and Luffy fanfiction could be about their adventures in a parallel universe. In this story, they might face different versions of their enemies and have to adapt their powers and strategies. Another could be a story where Ace survives and they continue their journey together to find the One Piece, strengthening their bond even more as brothers.
One common theme is friendship. Fanfictions often explore how female Luffy and Ace form a deep and unbreakable bond. They support each other through thick and thin in the dangerous pirate world.
One common theme is brotherly love. Since they are brothers in the One Piece universe, fanfics often explore the depth of their love and loyalty for each other. Another theme is growth. They both grow as characters, and fanfics may show how their relationship helps in that growth. For example, Luffy might become stronger because of Ace's influence.
One great 'Luffy visits Ace' fanfiction could be the one where Luffy travels to Ace's childhood memories. It shows their bond forming from a very young age. Another might be the one where Luffy visits Ace in a dream - like world, and they have deep conversations about their dreams and the meaning of family.
One common theme is brotherhood. Since they are like brothers in the original story, fanfics often explore and strengthen this bond. Another theme is adventure. They are all adventurous characters, so fanfics usually involve them going on exciting quests.