The Akatsuki could be potential enemies. They are always on the lookout for strong ninja to either recruit or eliminate. Since Kakashi is a well - known and powerful ninja, his little brother might draw their attention. They could see him as a potential threat or a valuable asset.
Some rogue ninja from other villages might target him. If word gets out that he is Kakashi's little brother, they might think that by capturing or killing him, they can get revenge on Kakashi or use him as a bargaining chip. For example, ninja from a village that Kakashi had defeated in a previous battle might seek to harm his little brother.
Perhaps the fanfiction could focus on a mission where Kakashi's little brother has to prove himself. He could be part of a team that is in a dangerous situation, and he has to use his unique skills, which might be different from Kakashi's. For example, while Kakashi is known for his Sharingan and lightning - style jutsu, his brother could be more proficient in genjutsu or some other form of combat.
The Akatsuki could be potential enemies. They might see Naruto's little sister as a way to get to Naruto. Maybe they think they can use her as a hostage or extract some information from her about Naruto or the Nine - Tails. Another enemy could be some of the rogue ninja who are after the bounty on Naruto's head and see his sister as an easy target to get to him.
One major plot point is the adjustment of their relationship. Naruto might initially be shocked or overjoyed to find out Kakashi is his brother. Then they have to figure out how to act around each other. Another plot point could be the impact on their respective goals. Naruto's goal of becoming Hokage might be influenced by having Kakashi as his brother, and Kakashi's role in the village and his own personal missions could also change due to this new family connection.
One popular 'naruto is kakashi's little brother fanfiction' might be 'The Hidden Bond'. In it, Naruto's relationship with Kakashi as his brother adds a new layer to the story. It shows how Kakashi would be more protective and how Naruto could learn different skills from him in a more family - oriented way.
The First Order could still be a threat. They might see her as a symbol of the Resistance and try to eliminate her. Or it could be a new Sith cult that has risen in the ashes of the old Sith order.
A new dark wizard could rise. This wizard might be jealous of the fame and power that Harry has, and seeing his daughter as an extension of that, decides to target her. Maybe this new dark wizard is seeking to gain control over the wizarding world and sees her as a key obstacle in his path.
There could be a new dark wizard or witch who is jealous of the Potter family's legacy. They might target Harry Potter's daughter to try and bring down the family name and prove their own power.
Danzo might be an antagonist. He's always been power - hungry and might see Naruto as a threat, especially if Kakashi is guiding and protecting him. Danzo could try to undermine Kakashi's efforts to raise Naruto and might even send his root agents to spy on them or cause trouble.
It could be some rogue ninjas who target Sakura because of her relation to Kakashi. They might think they can use her to get to him or extract information from him.
There are indeed fanfictions with Naruto being Kakashi's brother. Fanfiction writers like to play around with the established characters and relationships in the Naruto series. This particular concept can open up a whole new set of storylines. For instance, it could change the dynamics between Naruto and other characters like Sasuke or Sakura. Also, it might give Naruto a different perspective on the ninja world, as he would be related to someone as respected as Kakashi.