One possible plot could be that Akainu attacks the Straw Hat's new island, endangering Luffy's friends. Luffy, filled with rage, challenges Akainu. After an intense battle, Luffy discovers a new form of his Gear ability that allows him to withstand Akainu's magma and finally defeat him.
Considering Luffy's character development, he has always been against those who harm his friends and the innocent. Akainu has caused great harm, especially during the Marineford War. If Luffy continues to grow in strength, and if the story progresses in a way that pits them against each other again, there's a chance. But the Marines also have their own ways of protecting their top - ranked officers, so it's a complex situation.
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 common theme is the clash of ideologies. Akainu has a very strict and uncompromising view of justice in his original story, while Naruto has a more forgiving and rehabilitative view. So when they interact in fanfiction, this clash often forms a central theme.
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.