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 popular garp fanfiction story could be about Garp's early days in the Marines, his training and the friendships he formed. Another might focus on his relationship with his family members in a different light, like how he interacted with his grandson Luffy during Luffy's childhood. There are also stories that might explore Garp's role in some of the major events in the One Piece world from a different perspective.
One of the main themes is the search for identity. Garp is constantly trying to figure out who he is in the complex world around him. Another theme is the role of family. His family relationships, including his relationship with his mother, wife, and children, are full of love, conflict, and growth. Also, the idea of gender roles is explored, as Garp interacts with strong female characters and navigates through a society with certain expectations of men and women.