In 'luffy vs kaido fanfiction', Luffy's growth and new abilities are often explored. He might develop new forms of his Gear powers to face Kaido. Kaido, being extremely powerful, forces Luffy to push his limits. Fanfictions may also focus on the intense battle scenes, with detailed descriptions of their attacks and counter - attacks.
Another plot twist might be that Luffy actually loses the first round of the fight but then finds an inner strength or a new power - up from an unexpected source, like a memory of Roger or a new Devil Fruit ability that he unlocks in a moment of desperation.
I don't have enough context to determine exactly who 'garp' is. It could be a newly created character by the fanfic writer, or it might be a character from a relatively unknown story that is being incorporated into the 'ace fanfic'.
I'm not entirely sure specifically what 'garp save ace fanfiction' is about without more context. It could potentially be a fan - made story in which Garp tries to save Ace within the context of a particular fictional universe, perhaps related to One Piece.
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.