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.
Well, in this fanfiction, Harry might be a professor because of his extensive knowledge and experience in magic. He could be teaching younger students and sharing his wisdom.
One great Naruto and Harry Potter crossover fanfiction could be where Naruto is somehow transported to the wizarding world and meets a Professor Potter. In this kind of story, Naruto might use his ninja skills in Hogwarts, like using his chakra to perform amazing feats that baffle the wizards. He could team up with Professor Potter to fight against dark forces that are threatening both the ninja world and 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.