Perhaps there is a forbidden jutsu involved. Naruto accidentally discovers or is framed for using a very dangerous jutsu. Team 7, out of fear for the safety of the village, turns on him. Haku, being an agent of some shadowy organization, encourages this betrayal. He might convince Team 7 that Naruto is too unpredictable with this new power and that the only way to protect the village is to get rid of him. Naruto would then be left alone, having to fight not only his former friends but also the forces that are trying to use this situation for their own gain.
In this fanfiction, it might be that a misunderstanding is at the core. The village elders could spread false rumors about Naruto having some sort of dark power. Team 7, believing these rumors, decides to side against Naruto. Haku, who has his own motives related to the power struggle in the ninja world, takes advantage of the situation. He could manipulate Team 7 into thinking Naruto is a danger and needs to be stopped, which would be a heartbreaking betrayal for Naruto.
One possible plot could be that Team 7 is manipulated by a powerful enemy. Maybe Orochimaru manages to cast a powerful genjutsu on them, making them believe Naruto has turned evil. So they turn against him. It could also be that they are forced to choose between Naruto and the safety of the village in some extreme situation and they wrongly choose to sacrifice Naruto.
Naruto would probably be extremely hurt and confused at first. He would try to talk to his teammates, to understand why they are doing this. He might plead with them, asking them to remember their past together. But when he realizes they won't listen, he could become angry. Naruto has a strong will, so he would likely decide to prove his innocence on his own, vowing to show Team 7 that they are wrong.
The fanfiction could start with Haku in a life - threatening situation, perhaps captured by an enemy who wants to use Haku's unique abilities for evil. Naruto, being the hero he is, decides to sacrifice his own chakra, which is his life energy in the Naruto universe. He slowly gives up his chakra to keep Haku alive. This sacrifice could have far - reaching consequences, like Naruto losing his ability to use certain jutsus or even his life. And the story could then focus on how the other characters react to this selfless act of Naruto and how Haku copes with being saved at such a great cost.
One possible plot could be that Haku survives her encounter with Naruto and his friends. As they continue on their journeys, they keep crossing paths and gradually develop feelings for each other. Eventually, they decide to get married to bring peace between their respective clans or groups.
One possible plot could be that Haku somehow survives and through a series of strange events, ends up taking care of Naruto as his mother. Maybe she uses her skills to protect Naruto from early threats in the village.
One possible plot could be that Naruto discovers Haku is still alive after some hidden jutsu was used on her. They fall in love gradually and get married. Then they face the challenges of rebuilding Haku's clan together while also dealing with the political unrest in the ninja villages.
It could also be due to outside influence. An enemy might deceive Naruto's team into believing that Naruto has turned against them. The enemy could plant false evidence or use mind - control techniques on the team members, causing them to turn on Naruto without realizing the truth.
One possible reason could be jealousy. Maybe Sasuke in the fanfiction becomes jealous of Naruto's growing power and decides to betray him for his own gain. Another reason might be a misunderstanding. For example, if Naruto is wrongly accused of something terrible, and the rest of Team 7 believes it without giving him a chance to explain, they could turn on him. It could also be due to outside influence. Perhaps an enemy manipulates Team 7 into believing that Naruto is a threat to them, leading to their betrayal.
In this fanfiction, perhaps Naruto's neglect leads him to a desolate place where he meets Haku. Haku, having her own experiences of being an outcast in a way, understands Naruto. They form a partnership where Haku helps Naruto discover his hidden powers. Naruto, in turn, gives Haku a sense of belonging. They might embark on a journey to prove Naruto's worth to the village and along the way, their relationship deepens into a romantic one.
One possible plot could be that Naruto and Haku are in a situation where an extremely powerful enemy attacks. Naruto, being the selfless hero, jumps in front of Haku to take a fatal blow. Haku is then left with the guilt and determination to carry on Naruto's will. She might vow to bring peace to the ninja world, just as Naruto had always dreamed. This could lead to Haku joining the Hidden Leaf and trying to bridge the gap between different ninja clans in Naruto's memory.