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.
One great fanfiction could be 'Teddy's Growth' which shows Teddy Lupin's journey through Hogwarts with Harry as a sort of mentor figure. It really delves into their relationship and how Teddy copes with being a werewolf, just like his father. Another one is 'Harry and Teddy's Adventures' where they go on various magical quests together, solving mysteries and facing dark wizards. It's full of action and their bond is beautifully portrayed.
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.
They are popular because Harry has a connection to Teddy through his father, Remus Lupin. Harry can act as a father figure to Teddy, and this relationship has a lot of potential for emotional depth. It's a way to explore the idea of family and mentorship in the wizarding world.
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.