He could fit in as a hero. Just like in his own world, he would fight against the evil forces in Final Fantasy. For instance, if there was a dark sorcerer threatening the land, Naruto would step up to protect the innocent. His ability to make friends easily would also help him gather allies from different parts of the Final Fantasy world.
Naruto might start as an outsider but soon become integral to the story. He could get involved in the quests for crystals or the fight against the summoned monsters. His training as a ninja would give him an edge in combat. He could use his Sage Mode to sense the presence of hidden enemies or treasures. And his positive energy would attract those in the Final Fantasy world who are on the side of good, leading to new story arcs and alliances.
He might use his ninja skills in a unique way. For example, his Shadow Clone Jutsu could be used for various tasks like exploring dungeons simultaneously or confusing the enemies in the Final Fantasy world which often have complex battle systems. Also, his Rasengan could be a powerful attack against the magical creatures there.
They could be brought into the world through a magical portal. Naruto's energy and positive attitude would be a contrast to the often - darker tone of FF7. Gaara, with his calm and powerful presence, could be seen as an ally by some of the FF7 characters. They might start by helping the people of Midgar, using their ninja skills to deal with the Shinra soldiers or the various monsters that lurk in the slums.
Well, Naruto himself could fit in as a powerful ally. His speed and strength would be very useful in the battles against the daemons. He could also teach the characters in Final Fantasy 15 about perseverance and never giving up, which is a big part of his character. For instance, he could inspire Noctis to be more confident in his role as the king.
The ninja villages might be replaced by the different cities in Spira. The concept of the Hokage could be transformed into something like a high priest or elder in Spira's religious hierarchy. The Chunin Exams could be replaced with a Blitzball tournament or a pilgrimage to gain power. And the shinobi tools could be replaced with Final Fantasy 10 weapons and magic.
The characters from Final Fantasy would bring their unique skills and personalities. For instance, Tidus from Final Fantasy X with his blitzball skills could be like a new form of entertainment in Westeros. His positive and somewhat naive personality might make him an easy target for some of the more cunning characters at first. But his determination and fighting skills would also earn him respect. He could get involved in local disputes and use his skills to help those in need, while also trying to understand the complex power dynamics of the Seven Kingdoms.
The storylines can be blended by introducing the concept of chocobos in the Naruto world. Imagine Naruto riding a chocobo instead of using his normal means of transportation. Also, the idea of the Cie'th from Final Fantasy 13 could be incorporated into the Naruto world. Maybe some ninja are cursed and turn into Cie'th - like beings. As for the plot, it could start with a rift between the two worlds that causes chaos and forces the characters to work together to close it.
He'd bring unique combat skills. Naruto's ninjutsu would be a novelty in FF12's world. His ability to jump high and move quickly would be useful in battles. Also, his positive attitude. Naruto is known for his optimism, which could lift the spirits of the characters in FF12.
In a fanfic, Naruto's godlike nature might make him a key figure in understanding the secrets of the Lifestream. He could use his chakra sensing abilities, which are enhanced in his godlike form, to explore the Lifestream in ways that the characters in FFVII can't. He might discover new aspects of the Lifestream that could be used to either stop or control Sephiroth. This would add a new layer of mystery and power to the story.
He would fit in as a very powerful and unique character. His godlike powers would make him stand out among the other characters in the harem and in the Final Fantasy - inspired world.
Some fanfictions might focus on the backstories of minor characters. For instance, a story about a Moogle who dreams of becoming a great wizard. He starts on a journey to find a powerful crystal that is said to grant magical powers. Along the way, he meets all sorts of characters from the World of Final Fantasy and has to overcome many challenges to reach his goal.