Well, it could be about Naruto suddenly discovering a hidden talent for illusions. Maybe he starts to outdo Itachi in creating the most mind - boggling genjutsu. He could use these illusions not just in battles but also to solve some of the village's internal problems, like uniting feuding clans through the power of his illusions.
It could be a rogue ninja who is also a master of illusions and wants to use his powers to take over the ninja world. He might see Naruto as a threat to his plans since Naruto is developing his illusion skills.
Obviously, Harry Potter is one of the main characters as it's about his mastery of illusions. His friends like Hermione and Ron are likely to be important as well. They would probably support Harry in his journey to master this new skill and use it for good.
In some fanfictions, Harry's mastery of illusions might lead to him being able to enter people's dreams and manipulate them. He could use this power to either help his friends overcome their fears or to gather information from the enemy. However, he would also have to face the moral implications of such actions.
Perhaps an organization that views the 'god of illusions' as a threat to their plans for world domination. They might see the 'god's' influence over illusions as something that can disrupt their schemes. So they send their most powerful agents to take on the 'god of illusions'. This could lead to some epic battles in the fanfiction.
Well, in this fanfiction, perhaps a really cool concept is the origin story of the 'god of illusions'. It could be that this entity has a deep connection to an ancient and forgotten clan in the Naruto world. Also, the idea of the 'god of illusions' being able to manipulate not just what people see, but also their emotions and memories through illusions is quite fascinating.
The kitsune illusions make Naruto more mysterious. Instead of just relying on his regular ninja skills, he now has this otherworldly power. It gives him an edge in battles as his opponents are often caught off - guard by the illusions. For example, he can create illusions of himself that can attack from different angles, making it difficult for the enemy to counter.
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.
He could start by training intensively with different weapons under the guidance of some old and experienced ninja. They might teach him the proper techniques and stances for using each weapon effectively.
Yes, there are likely fanfictions out there where Naruto is depicted as a poison master. Fanfiction writers often like to take established characters and put them in unique and unexpected roles. They might explore how Naruto's skills and personality could be adapted to the role of a poison master, perhaps using his knowledge of herbs from his time in the forest or his ninja stealth to create and administer poisons in a creative way.