Maybe he is exposed to some dark and powerful jutsu that corrupts his mind. This jutsu could make him lose his empathy and sense of right and wrong. He would then view life as disposable and become a cold - blooded killer. In the fanfiction, it could be a forbidden jutsu that was hidden away but somehow he discovers it and it changes him completely.
One possibility could be a great tragedy that befalls him. For example, if all his friends were brutally killed in front of him by an enemy he couldn't defeat at that time. This might shatter his belief in the goodness of the world and drive him to become cold - blooded in order to seek revenge. He would start to see killing as the only way to get justice.
In the fanfiction, the plot could revolve around Naruto being influenced by a powerful and evil organization. They manipulate him into believing that the only way to bring peace is through force and killing. As he becomes a cold - blooded killer, we would see him going against his former comrades. There would be intense battles where Naruto uses his new - found ruthless tactics. His character arc would be a dark descent from a hopeful hero to a feared killer.
Well, one way could be through some kind of strange jutsu gone wrong. Maybe Naruto was experimenting with a new type of chakra manipulation and it accidentally combined with some sort of mutagenic energy, similar to what created the Hulk. And then, poof! He starts turning into the Hulk whenever he gets angry. Another possibility is that he encounters a villain who has the power to transform others and decides to use Naruto as his guinea pig to create a super - strong and uncontrollable Naruto - Hulk hybrid.
Well, in fanfiction, anything is possible. Maybe Naruto gets transported to the Sekirei world through some sort of magic or a strange portal. This could be the start of his journey to become a Sekirei. It could also be due to a curse or a wish gone wrong that transforms him into a Sekirei - like form.
Well, in a fanfiction scenario, Naruto could become Kazekage if there was some sort of political alliance between Konoha and Suna. Maybe after years of working together on various missions and sharing resources, the two villages decided to have a more unified leadership. Naruto, being a well - known and respected ninja, could be chosen to lead Suna as Kazekage. He has shown his ability to understand and work with different people, which would be crucial in leading a village not his own.
If Naruto loses all his friends in a tragic way, perhaps in a battle against an overwhelming enemy that the village could have prevented. His grief could turn into hatred towards the shinobi system. He may decide that the existing order is what causes all this pain and destruction, and then he starts to work against it, becoming a villain in the eyes of the village.
In a Naruto - becomes - a - Yonko fanfiction, Naruto's unique abilities would set him apart. For example, his Sage Mode gives him enhanced strength, speed, and perception. In the pirate world, this would make him a formidable opponent. He could also learn Haki, the power in the One Piece universe. Combining his ninja skills with Haki, he could easily take on other pirates and claim territories, eventually becoming a Yonko.
In a Naruto becomes a Terminator fanfiction, it could start with some sort of time - travel or interdimensional rift. Maybe a jutsu gone wrong sends Naruto to a world where the Terminator technology exists. Then, Naruto, with his ninja skills and strong will, might adapt to the new environment. He could use his chakra in unique ways combined with the advanced weaponry of the Terminator world.
It's possible that Natsu's coldness is a temporary state. He could be under a spell that makes him seem cold, but deep down his true self is still there. As the story progresses, his friends might try to break the spell and bring back the old Natsu. They could search for a cure or a way to reverse the magic, leading to many adventures and challenges along the way.
Imagine a situation where the Akatsuki has a new member who has the ability to show illusions of other powerful beings from different universes. This new member shows Naruto the story of Thanos. Naruto, seeing Thanos' determination and his ability to amass so much power single - handedly, could get influenced. He might start to think that there are things he can learn from Thanos about achieving one's goals, no matter how extreme, and thus become a fan.
Another possibility is that Naruto's will and his existing skills attract the attention of a dragon spirit. Naruto is known for his determination and his ability to master difficult techniques. A dragon spirit, seeing his potential, could choose to merge with him or guide him on the path to becoming a Dragon Sage. This might involve him undergoing a series of trials, like facing his inner demons and proving his worthiness to wield the dragon - sage power.