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.
One great fanfiction could be about Remus Lupin teaching Harry Potter some advanced defensive magic techniques, with Sirius Black popping in now and then to offer his unique insights. It might explore how Remus's calm and methodical approach contrasts with Sirius's more impulsive nature, all while guiding Harry on his journey to become a better wizard.
There are fanfictions where Lupin, Black, and Potter form a sort of secret alliance. In these stories, they might be investigating a mystery within the wizarding world that the Ministry of Magic is trying to hide. For example, they could be looking into the true nature of a new dark magic spell that's been causing chaos. Lupin's werewolf knowledge, Black's family secrets, and Potter's natural heroism all come into play.
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.