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'.
One idea could be a crossover with the One Piece world where Kuzan teams up with the Straw Hat Pirates to face a new and powerful enemy. Maybe they encounter a threat that requires Kuzan's ice powers and the Straw Hats' unique skills combined. Another could be a crossover with a modern - day superhero universe, and Kuzan uses his abilities in a city - scape setting, like freezing over a rampaging villain's lair.
Begin by brainstorming the characters Kuzan will interact with in the crossover. If it's a crossover with the Star Wars universe, think about how he would get along with Jedi or Sith. You could start by writing a short prologue that shows Kuzan in his original One Piece world and then something that causes him to be transported to the Star Wars galaxy. After that, develop the story by having him encounter different species and use his ice powers in unique situations.
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.