It could also change his stealth abilities. Since it's a shadow weapon, it may allow Naruto to blend into shadows more easily, making it easier for him to spy on enemies or launch surprise attacks. This would be a great advantage in his ninja missions.
The shadow weapon might be an ancient relic from Wave. Maybe it was created by a powerful shinobi from Wave long ago and has been lost until Naruto found it.
If the weapon is a chakra - enhancing tool, it will boost Naruto's existing jutsu. For example, his Rasengan could become more powerful, being able to break through stronger defenses.
Well, it could be a key to unlocking new powers for Naruto. Maybe it has some special chakra - enhancing properties that help him in battles against stronger foes.
It might be a set of shuriken, but not the ordinary ones. These shuriken could be larger and have a unique shape, like the outline of a wave. They could also be made of a special metal that's resistant to most forms of damage and has the ability to return to Naruto's hand like a boomerang.
Perhaps it's a unique staff. A staff could have different uses, like being a conduit for chakra. It could also have hidden compartments or be made from a rare material that gives Naruto an edge in battles. For example, it might be made from a metal that can absorb chakra attacks and then redirect them.
If the weapon is a blade, it will probably make his taijutsu more lethal. He can combine his already impressive speed and strength with the weapon's sharpness. His attacks will be more precise and deal more damage.
Well, it could be a sword with the power to cut through any jutsu. Maybe it has some special markings that only Naruto can understand. It might also be a staff that can control the elements in a unique way, giving Naruto an edge in battles.
He could start by training intensively with different weapons under the guidance of some old and experienced ninja. They might teach him the proper techniques and stances for using each weapon effectively.
His friends might be a bit wary of the sentient weapon at first. Sasuke, for instance, might be suspicious and think it could be a danger. But Naruto would try to convince them that it's okay. Over time, if the weapon proves useful, they could come to accept it.
He would be more cold - blooded. Instead of the warm - hearted Naruto we know, he'd be focused on combat and following orders. His personality would lack the playfulness and the drive to make friends.