It could be Naruto himself. He might be trying to create new weapons to better fight his enemies. Another possible main character could be Sasuke. He has a great understanding of weaponry and chakra, so he would be interested in making unique weapons. And then there's also Kakashi. His knowledge and experience would make him a great asset in the weapon - making process in the fanfiction.
Naruto could also make a staff. Given his connection to nature energy through Sage Mode, he could fashion a staff that can better conduct and store nature energy. It could be made from a special type of wood found in the Hidden Leaf's forest, which has natural chakra - absorbing properties.
The katana would be really interesting. A katana in Naruto's hands could be a powerful weapon. He could use his speed and strength to make swift and deadly strikes. Also, a chained weapon like a kusarigama. He could swing it around with great force and use his ninja skills to control its movement in battles, entangling his enemies.
He could become a weapons master through some kind of special power-up. For example, a new chakra form that gives him an affinity for weapons. He might be able to infuse his chakra into weapons, making them more powerful. And Naruto is known for his hard work, so he'd surely spend countless hours training with various weapons, from swords to shurikens, until he masters them.
One way is to base the weapons on existing ninja tools in the Naruto universe. For example, you could create a modified version of the kunai, like a kunai with a hidden blade that can extend for surprise attacks. Another idea is to draw inspiration from nature. Maybe a weapon made from a special tree in the Hidden Leaf Village that has unique properties, such as being able to absorb chakra.
I recommend the following fantasy novels where the protagonist can create weapons:
1. [Blacksmith Master]: It tells the story of a talented young man who was born with a fire in his heart. He searched for divine weapons, forged divine weapons, and finally became a Blacksmith Master of his generation.
2. " I Forge a Blade in the Demon Slayer Division ": The protagonist is a blade forger who specializes in forging weapons for Demon Slayer Guards. He can add characteristics to various weapons and obtain the ability to defeat demons.
3. The main character of the story was an old craftsman. He adopted a child who was about to freeze to death from the street. Through forging weapons and practicing martial arts, he stepped on the path to immortality.
I hope you will like the novels recommended above!
In this fanfiction, Naruto might also create a shield. This shield could be made from a combination of rare materials he discovers during his adventures. It has the ability to not only block physical attacks but also absorb and redirect chakra - based attacks back at the enemy.
Another possible plot is that Naruto discovers a hidden scroll in the Uzumaki clan's ruins. The scroll contains the secrets of making SS weapons. He spends time deciphering it and gathering the necessary ingredients. Along the way, he has to face off against other ninja clans who also want the secrets. Once he creates the weapon, it gives him the ability to defeat enemies that were previously unbeatable, like a weapon that can nullify any genjutsu and also boost his physical attacks at the same time.
Some fanfictions describe that Naruto chakra weapons are created by skilled blacksmiths who have a basic understanding of chakra. They infuse chakra - conductive materials with the user's chakra during the forging process.