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'.
Blackbeard would probably be very interested in the magic in the Fairy Tail world. He might try to steal some of the powerful magic items or learn magic for himself. He could see the mages as a threat to his plans if they oppose him, but also as potential allies if he can manipulate them. He would likely use his cunning and power - hungry nature to his advantage in this new world.
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.