It could be Danzo. He was always power - hungry and had his own ideas about what was best for the village. He might see Naruto as a threat and find a way to take away his birthright.
In such a fanfiction, perhaps Naruto could be wrongly accused of something that leads to him being stripped of his birthright. Maybe a false claim about his connection to the Nine - Tails is used against him.
One possible consequence could be a change in his fighting style. Without his eye, he might have to rely more on his other senses like hearing and smell, similar to how blind ninja in the Naruto universe adapt. For example, he could focus more on using his chakra to sense the movements of his opponents.
Naruto might also go through an identity crisis. His whole life has been centered around being a ninja with chakra and using powerful jutsu. Losing his chakra could make him question who he is and what his purpose is. He would have to redefine himself and find a new way to contribute to the ninja world. This could be a long and difficult process for him.
Maybe he starts to train with one - armed techniques. He could learn from other one - armed ninjas if there are any in the fanfiction world. His friends would also play a big role. They might encourage him to keep going or help him in his training.
In this fanfiction, there could be a villain who has the ability to manipulate Naruto's mind. The villain might use this power to make Naruto turn against the Hidden Leaf Village. Naruto, without control of his own mind, could start causing chaos within the village, like attacking important buildings or trying to steal important ninja secrets.
In this fanfiction, Naruto could lose his way by being separated from his friends for a long time. Without their support and guidance, he may make wrong decisions. For example, he could be tricked into working for an enemy who disguises as a friend in need. This would lead him down a path where he gradually loses sight of his original goals and the positive ninja way he once followed.
Well, perhaps he loses his identity due to some sort of dark magic or a powerful enemy's jutsu. As a result, he could end up on a journey to rediscover himself. He might have to face many challenges without the confidence that his identity once gave him, like having to fight enemies without his well - known combat style.
One possible reason could be for Naruto's protection. His parents might think that by giving away his birthright, they can keep him away from dangerous enemies or political intrigue in the ninja world. Maybe they foresee a great threat that would target Naruto specifically because of his birthright, and this is their extreme measure to ensure his survival.
A new, unknown villain could be the cause. This villain might have a special power related to mind control or mental manipulation. They target Naruto because of his status as a hero in the ninja world. By making Naruto lose his mind, they could throw the entire ninja world into chaos.