Perhaps Naruto will gain the power to manipulate the energy of death. He could use it in combat, for example, to weaken his opponents by sapping their life force or to create barriers made of death energy that are difficult to penetrate. Another ability could be to communicate with the spirits of the dead. This could give him access to knowledge and wisdom that no one else has, helping him in his battles and in his journey to protect the village.
He could gain the ability to sense death. Maybe he can tell when someone is close to dying or if there's a dangerous situation that could lead to death. It would be like a sixth sense.
One unique ability Naruto might get is the power of resurrection, in a limited sense. He could bring back someone from the brink of death for a short period, just long enough to get important information or to save them from a final, fatal blow. Also, he might gain the ability to move at super - fast speeds in the realm of death. This would allow him to avoid attacks that are otherwise impossible to dodge in the normal world, and he could use this ability to surprise his enemies during combat.
In fanfiction, Naruto could gain enhanced sensory abilities. Since toads are known for their connection to nature, Naruto might be able to sense things from a greater distance, like the movements of enemies or the flow of chakra in the environment. This would give him an edge in battles.
He could potentially gain an enhanced understanding of wood - style techniques. Since Hashirama was a master of it, Naruto might learn how to create more complex and powerful wood constructs, like huge forests for both offense and defense.
He might gain better chakra control. Kakashi could teach Naruto how to precisely manage his chakra, allowing him to perform jutsus more efficiently and for longer periods. For example, Naruto could use his Shadow Clone Jutsu without exhausting his chakra so quickly.
He might gain enhanced spiritual powers. Kami could teach Naruto to better sense and interact with spiritual energy, which could give him an edge in battles against opponents with spiritual - based attacks.
He could gain better stealth skills. Hidan is quite good at sneaking around and Naruto might pick up on that. So he could move more quietly and be better at surprise attacks.
Naruto could gain the ability to use different forms of his Nine - Tails chakra more effectively. Since Killer Bee has great knowledge about Jinchuriki powers, he could teach Naruto how to access and use his chakra in ways he hadn't thought of before. Also, Naruto might learn new physical combat techniques from Bee, like improved hand - to - hand combat skills or new ways to use his kunai and shuriken.
He might gain enhanced chakra control. Since Kami is a powerful entity, Naruto could learn to manipulate his chakra in ways he hadn't before, like creating more complex jutsus with less effort.
Naruto would gain super strength. He could lift extremely heavy objects with ease, like mountains or large buildings in the Naruto world. He would also have super speed, enabling him to move faster than any ninja technique. And of course, he would be able to fly, which is a completely new ability in the Naruto universe.
Naruto could gain the ability to manipulate light and darkness, which are often associated with Keyblades. This would be a huge change from his normal chakra - based abilities. He could also use the Keyblade to fly, giving him an advantage in both mobility and combat. Another new ability might be the power to heal, as some Keyblades have restorative properties.
Naruto might gain superhuman strength like Kratos. He could easily break through solid rock or lift extremely heavy objects. His speed could also increase, allowing him to move so fast that enemies can barely see him. Maybe he could also have the ability to summon the blades of chaos, which he could use in combat in a unique way, like combining them with his kunai.