He could be a powerful antagonist. Picture him arriving in the Naruto world with his otherworldly powers. His sword skills and magic would make him a formidable foe for Naruto and his friends. He might be interested in obtaining some of the unique powers in the Naruto universe, like the tailed beasts' chakra.
Sephiroth could also be a mysterious ally. Maybe he has his own reasons for being in the Naruto world and decides to help the shinobi against a greater threat. He could share some of his knowledge of combat and power with the characters, though his motives would always be a bit suspect.
The integration could start with Naruto's backstory. Maybe he's a being with a natural affinity for the Force, but his powers manifest in a more ninja - like way. In the fanfic, he could meet characters like Luke Skywalker or Obi - Wan Kenobi. They would be intrigued by his Sage abilities. Naruto could then teach them about chakra and in return, learn more about the Force. This exchange of knowledge could lead to new forms of combat and strategies in the Star Wars universe.
Sephiroth might possess the power to manipulate the elements in a different way compared to Naruto characters. For example, instead of using hand signs for fire jutsu, he could summon dark - tinged flames just by his will. His power over the Lifestream could be reimagined as a connection to a different kind of energy source in the Naruto universe.
I haven't read this fanfic specifically. But generally, if Naruto is the son of Sephiroth in this fanfic, it would be a very interesting crossover concept. It might combine the ninja world of Naruto with the unique elements from the world where Sephiroth exists, like different powers and cultures.
It could be a comical take in the fanfic as well. Maybe a crazy inventor or a bumbling wizard accidentally casts a spell that merges the two worlds in a strange way, making Sephiroth Naruto's brother. This would lead to hilarious situations like Sephiroth trying to understand Naruto's love for ramen or Naruto being baffled by Sephiroth's long, flowing hair and his rather dark and brooding personality.
Naruto would probably be very curious about Sephiroth. He'd likely try to be friendly at first, asking a lot of questions. Sephiroth, on the other hand, would be more aloof and might see Naruto as a nuisance or a potential threat. But as time goes on, Naruto's determination might start to intrigue Sephiroth.
Naruto's character would likely become more brooding. Sephiroth's influence could make Naruto more introspective and less likely to wear his heart on his sleeve. He might develop a colder exterior to match Sephiroth's aloofness.
Naruto's relationship with his sensei, like Kakashi, would also change. Kakashi would be extremely vigilant and try to train Naruto to control the Sephiroth - like powers. He might also be researching about this strange phenomenon. As for his friends in the village, some might be afraid of him, while others might see it as an opportunity for the village to become stronger. But overall, Naruto would feel a bit isolated as he's different now.
I'm not sure specifically as I haven't read that exact fanfic. But generally, it might be a story where Naruto, who is in a godlike state, has some sort of connection or interaction with Sephiroth. It could involve Naruto taking on Sephiroth's characteristics or the two of them being in a strange new world together.
His personality might become more brooding. Sephiroth has a rather dark and mysterious personality, and some of that could rub off on Naruto. Naruto might start to keep more to himself and not be as open with his emotions as he used to be.
Naruto's character would become more serious. Sephiroth's training is no joke and Naruto would have to focus more. He might lose some of his naivete and become more worldly.