In a fanfiction, Ace's own experiences and battles could be related to Luffy's metabolism. Maybe Ace has some knowledge about a special fruit or power that could enhance Luffy's metabolism. He could share this with Luffy, leading to a new adventure where Luffy tries to obtain this thing to boost his metabolism. Also, Ace's death could have an impact on Luffy's metabolism. Luffy could go into a state of shock or extreme grief that either slows down or speeds up his metabolism in an unexpected way.
Well, first of all, Luffy is a very emotional character. When he discovers Zoro's sacrifice, he would probably go into a state of shock initially. Then, his reaction could be explosive. He might not be able to accept it right away and would try to find a way to reverse the situation. He could also start blaming himself for not being there to protect Zoro, which would further fuel his determination to become stronger and protect the rest of his crew better in the future.
Well, there is some 'naruto luffy brother fanfiction'. Some of them explore how their different powers would interact if they were brothers. For example, Naruto's ninjutsu and Luffy's Devil Fruit abilities could make for some really exciting adventures together. Also, the different worlds they come from, the Hidden Leaf Village and the Grand Line, could be combined in creative ways in these fanfictions.
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.
Whitebeard is another character that could be important in these fanfictions. As Ace's adoptive father, he plays a significant role in Ace's life. Fanfictions might explore how Whitebeard's influence on Ace affects his relationship with Luffy, like how Whitebeard's values of family are passed on to Ace and then reflected in how he treats Luffy.
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.
In some fanfictions, Ace's death becomes a turning point for Luffy's character development. It makes Luffy more determined and focused. Ace's legacy lives on through Luffy, and this is a common theme. Ace's values, like loyalty and protecting the weak, are also passed on to Luffy in these stories.
Their relationship started when they were young. Luffy was always chasing after Ace. At first, Ace was a bit annoyed by Luffy but gradually, they became inseparable. They had many experiences together on their small island. Later, even as Ace ventured out into the world with the Whitebeard Pirates, his connection to Luffy remained strong. He was always protective of Luffy, and this was clearly seen when he faced danger at Marineford to protect his younger brother.