One great 'Harry Potter - Arsene Lupin' fanfic is 'Lupin's Legacy'. It delves deep into Lupin's past and his relationship with the other characters. It shows how his werewolf nature affected his life at Hogwarts and beyond in a really interesting way.
Maybe in that fanfic, Lupin did something that Harry considered a betrayal. For example, he might have made a decision that put Harry or his friends in danger without consulting them first.
In such a fanfiction, perhaps Harry would grow up with a more nurturing and understanding environment. Lupin could teach Harry about the wizarding world in a very different way than the Dursleys. He'd likely introduce Harry to magic earlier and help him better understand his parents' legacy.
Character development makes it interesting. In a good fanfic, Lupin's complex character is explored further. His struggle with his werewolf identity, his loyalty to his friends, and his role as a teacher all can be delved into more deeply than in the original books.
Many 'Harry Potter Remus Lupin fanfiction' deal with his sense of isolation. Despite having friends, he often feels alone because of his condition. Writers explore how he overcomes this feeling or succumbs to it. His relationship with the wizarding community at large, whether they accept or reject him due to his lycanthropy, is also a common theme.
Some fanfictions explore Teddy Lupin's time at Hogwarts in a more emotional way. He could struggle with feeling different because of his werewolf heritage. Harry, having faced his own share of hardships, could be there to support him. These stories often show how Teddy grows as a wizard and a person, with Harry being an important influence in his life.
One possible event could be that Lupin lied to Harry about something important. In the world of Harry Potter, trust is a huge factor. If Lupin deceived Harry, it could easily turn Harry against him. Say, Lupin lied about his whereabouts during a crucial time when something bad happened, and Harry suspected Lupin's involvement.
Well, perhaps in the fanfiction, Harry adopts Teddy Lupin right after the Battle of Hogwarts. Teddy is just a baby then. Harry, who has lost so many people, sees Teddy as a new beginning. He might face challenges from the wizarding community as some may question his ability to raise Teddy. But Harry, with the help of his friends like Hermione and Ron, overcomes these difficulties and Teddy grows up to be a strong and kind wizard.