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 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.
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 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 could develop an enhanced sense of touch. For example, he could sense the vibrations in the ground when someone is approaching, just like how some real - world blind people can read by touching the raised dots on a Braille book.
He might have enhanced senses like a fox, such as better smell and hearing. This could help him in tracking enemies or sensing danger from afar.
He might have the ability to form a symbiotic bond with others like Venom does. So he could potentially share his powers or enhance the abilities of those he bonds with, like Yugito. He could also have enhanced physical strength, speed, and agility, similar to how Venom gives its host enhanced physical capabilities.
One unique ability could be enhanced sensory perception. He might be able to sense threats from much further away or even detect the true intentions of people. Another ability could be the power to teleport short distances instantly. This would give him an advantage in combat, allowing him to dodge attacks and surprise his opponents.
Naruto could develop the ability to better understand and manipulate the chakra of others. His maturity might give him the wisdom to see the flow of chakra in different ways. Additionally, he may gain the ability to quickly analyze battle situations and come up with more creative solutions due to his increased mental maturity.