They could go on a mission to find a rare magical creature that is important for some reason. Maybe it has a power that can help defeat the dark forces. Or they might travel to a hidden magical place that Lupin once knew about from his own adventures as a young wizard. There, they face various challenges and have to use their combined knowledge of magic to get out safely.
In many such fanfictions, Harry might start off with a basic admiration for Lupin as his teacher. But as the story progresses, he could come to see Lupin as a true father figure. Maybe Lupin shows him kindness and understanding that Harry lacks from other adults in his life. This could make Harry's view of Lupin deepen from simple respect to a more profound emotional connection.
One possible reason could be his self - loathing due to his werewolf condition. He might think he's not good enough for Tonks and that she'd be better off without him, fearing that his lycanthropy could bring danger or a difficult life to her.
Definitely Sirius Black. Since he was one of Lupin's best friends and also had a strong connection with Harry. His presence can add a lot of drama and backstory to the fanfiction.
One interesting fanfiction idea could be that Snape discovers a hidden Horcrux that Lupin unknowingly has in his possession. Harry Potter then has to figure out how to destroy it without harming Lupin. This would add a whole new layer of drama and mystery to the existing lore of the Harry Potter world.
One great fem Harry Potter and Remus Lupin fanfiction is 'A Different Path'. It explores an alternate universe where Harry is female and her relationship with Remus develops in a very interesting way. It delves deep into their characters and how they interact in a world full of magic and mystery.
I'm not entirely sure as I haven't read that specific fanfiction. It could be about Harry Potter having a new identity as Remmy Lupin for various reasons. Maybe it's a story exploring a different path for Harry within the wizarding world, or it could be a what - if scenario where he takes on Lupin's name for some kind of mission or adventure.
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.
At first, it would become very strained. Harry might avoid Lupin at all costs. He would no longer trust Lupin and would be suspicious of his every move.