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.
One possibility is that the sentient weapon has its own goals and motives. It could lead Naruto on a quest to fulfill something it desires. For example, it might need to be reunited with its original creator or find a special place. Naruto would likely go along with it, seeing it as an adventure and also because he trusts his instincts about the weapon not being evil right away.
I recall a fanfic called 'Sentient Library: Naruto's New Ally'. In it, the sentient library is a sort of magical entity that has been observing the ninja world for ages. When Naruto finds it, it offers him knowledge that was lost over time. It shows Naruto different historical events through magical books, which gives him an edge in battles and also in understanding the politics of the ninja villages.
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.
Well, he could gain new powers. Maybe the sentient library has books that teach him long - lost jutsu or give him knowledge about the nature of chakra that no one else in the ninja world knows. For example, he might learn how to combine different elemental chakra in a completely new way.
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.
The sentient sword could also have a tragic past. Maybe it was once used for evil and now wants to atone by helping Naruto. It could share its knowledge of ancient jutsu or forgotten battles with Naruto, which would give Naruto an edge in his future fights.
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.
The sword could make Naruto more cautious. Since it's sentient, it might have its own motives that Naruto doesn't fully understand at first. So he has to be more careful about his actions and decisions.
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.