A major challenge would be Harry's own internal struggle. He has always fought against dark forces and Fenrir has been associated with the dark side in the past. Harry would need to reconcile his past views with this new relationship. Additionally, they would have to face potential attacks from those who oppose the idea of a human - werewolf relationship. The wizarding society's norms and laws might also pose difficulties, as there may not be any provisions for such a unique relationship.
One challenge would be acceptance from the wizarding community. Since Fenrir is a werewolf, many wizards view werewolves as dangerous and uncontrollable. Harry would have to deal with the prejudice of his friends and colleagues towards his relationship with Fenrir.
They would face the challenge of Fenrir's nature. His werewolf side might be difficult to manage, especially during full moons. Harry would need to find a way to support Fenrir during those times without endangering others. Also, there could be dark forces that might try to use their relationship against them, seeing it as a weakness to exploit.
One possible plotline could be that Harry, in his 'evil' form, forms an alliance with Fenrir. They might decide to take over the wizarding world together, using Fenrir's werewolf strength and Harry's knowledge of magic. Another could be that they are on a quest to find a powerful magical artifact that can change the fate of the werewolves in the wizarding world.
One challenge could be his own reputation. Since Fenrir is often seen as a dangerous werewolf, other characters might not trust him around Hermione, which could make it difficult for him to carry out his protection duties effectively.
In these fan fictions, the theme of self - discovery is quite common. Harry, for example, discovers new aspects of himself through his relationship with Fenrir. Also, the theme of loyalty is present. They are loyal to each other despite the odds. Additionally, the exploration of a different kind of love, not the typical wizarding love, but a more primal and fated love is a common theme.
One challenge could be the difference in their houses. Cedric is from Hufflepuff and Hermione is from Gryffindor. Their house values might sometimes clash, causing misunderstandings. For example, Gryffindors are known for their boldness which might seem reckless to a Hufflepuff, and Hufflepuffs' loyalty to all might be misinterpreted as being too inclusive by a Gryffindor.
The secrecy that surrounds Itachi's past and his actions could be a major challenge. Fem Naruto might find it difficult to fully trust Itachi when she discovers parts of his past that are morally ambiguous. Also, Itachi's loyalty to the village in his own way might conflict with Fem Naruto's more straightforward ideas of justice, creating friction in their relationship.
One big challenge is their pre - existing relationship. They've had a lot of conflicts in school. Snape has been rather harsh on Harry, so they need to overcome that animosity. Also, Snape has to deal with Harry's trauma from the Dursleys. He needs to help Harry heal and grow in a positive way, which is not easy for someone as emotionally reserved as Snape.
In this fanfiction, they face multiple challenges. Firstly, the cultural norms of the ninja world. A relationship between a human, even a special one like fem Naruto, and a tailed beast like the male Kyuubi is quite unorthodox. People might shun them or try to separate them. Secondly, the Kyuubi's nature. He has a wild and powerful side that might be difficult for Naruto to handle at times. And Naruto, on her part, has her own insecurities and self - doubts, especially when it comes to being in a relationship with such a powerful being. These insecurities can cause friction in their relationship. Thirdly, external threats. Enemies might target them because of their relationship, seeing it as a way to either gain power or disrupt the ninja world order.
One common theme is loyalty. Harry and Remus are always there for each other. For example, in many fanfics, Remus will risk his life to protect Harry, just as Harry would do the same for Remus. Another theme is mentorship. Remus often teaches Harry about things he doesn't learn in school, like some of the more hidden aspects of the wizarding world related to werewolves.
I'm not sure of specific 'evil harry and fenrir mates fanfiction' off - hand. But you can try searching on fanfiction - dedicated websites like Archive of Our Own. There are likely to be many creative takes on this rather unique pairing in the Harry Potter fanfiction universe.
In the fanfiction, a big challenge could be the jealousy or interference from other characters. There might be other people who have feelings for Naruto or Hanabi. For Naruto, Sakura might still have some unresolved feelings, and she could potentially cause some problems for their relationship. For Hanabi, there could be members of the Hyuga clan who oppose her being with Naruto, thinking that he is not a suitable match for her in terms of clan traditions and politics. All these factors combined would create many challenges for Naruto and Hanabi to overcome.