The scene of Sukuna's death in the manga is a climactic moment. It's filled with action and drama, as various powers and tactics are employed to bring him down. The build-up to this point is full of tension.
Perhaps it could have a power of transformation. Not just in terms of its physical form, but also in how it affects Naruto. It could transform Naruto into a more spiritual - like being for a short time, granting him enhanced senses and the ability to interact with spiritual realms, which would be a very cool addition to his abilities in the fanfic.
Sukuna does die in the manga. The way it happens is quite dramatic and ties in with many other elements of the story, adding depth and tension to the overall narrative.
Maybe through some kind of special training or a unique ritual. It could be that he discovers a hidden method within the ancient scrolls of his village.
In this fanfiction concept, Naruto's powers would be a blend of his own and what he could inherit from Itachi. Since Itachi was a very powerful ninja, Naruto might have an innate ability for genjutsu, which was one of Itachi's strong suits. His chakra control could be improved due to his Uchiha blood. And with the Sharingan, he might be able to use Susanoo in the future, although it would probably be a different form compared to Itachi's, as Naruto also has the Nine - Tails' chakra. This combination of powers would make him an extremely formidable ninja.
He could develop the ability to combine his chakra with Superman's powers. For example, he might be able to use his Rasengan while flying at super - speed. Or he could infuse his chakra into his heat vision to make it even more powerful.
In this fanfiction, the prototype powers could be something like an early form of chakra manipulation that is different from the traditional Naruto world's chakra use. It might be more raw and unrefined, with the potential for new and unique jutsu.
Well, in the fanfiction, it's possible that one of the sisters has a power related to the wind, just like Naruto. But instead of using it for offensive attacks mainly, she uses it for more defensive and utility purposes, like creating wind barriers or using the wind to carry messages. Another sister could possess a fire - based power. She could be a master of fire jutsu, creating intense flames that can melt metals. The third sister might have a unique power of light, being able to blind enemies or use light to create illusions.