Maybe it has the ability to absorb chakra. When Naruto is in a fight, the sword can absorb the enemy's chakra when it makes contact. This would weaken the opponent and at the same time, give Naruto an extra boost of chakra that he can use for other jutsu or to strengthen the sword's attacks even more.
It could have the power to cut through almost anything. Since Naruto is a powerful ninja, the sword might inherit some of his chakra's strength, allowing it to slice through thick barriers or even powerful jutsu defenses.
In some fanfictions, the Buster Sword could be enchanted. So Naruto might gain abilities like being able to send out energy slashes from the sword. These slashes could travel long distances and be very powerful, capable of taking out multiple enemies at once. Also, it could have a special connection to Naruto's chakra, allowing him to channel his chakra into the sword more effectively and use it in new and creative ways during combat.
He might infuse the sword with his chakra. Since Naruto has a large amount of chakra, he could make the Buster Sword glow with his unique chakra color. This would not only make the sword stronger but also give it new abilities. For instance, the chakra - infused sword could cut through stronger defenses or even repel enemy ninjutsu.
Well, in some fanfictions, Naruto might get the Buster Sword through a special encounter. Maybe he stumbles upon it during a mission in an ancient ruin. Another possibility is that it could be given to him by a powerful ally who has connections to the world where the Buster Sword originally comes from.
Another possibility is that Naruto gets the materials as a reward. He could complete a really difficult mission for a blacksmith or a village elder. In return, they give him the unique and strong materials required to make the buster sword. He would then use his own creativity and some basic forging knowledge to shape it into the buster sword.
In fanfiction, the sword could have a spiritual power. It might be able to communicate with Naruto on a spiritual level, guiding him in battles or revealing secrets about the past. It could also have the power to create illusions specific to the Cloud Village's history or mythology, confusing enemies and giving Naruto an advantage in combat.
He could have the power of instant regeneration. So, no matter how severe the injury, Naruto could heal himself in a split second. This would make him extremely difficult to defeat in combat.
One idea could be Naruto finding the Buster Sword in a hidden ancient ninja cache. He then has to learn how to use it in his own unique style, combining his chakra powers with the brute force of the sword.
He could have the power to create poison clouds. With a simple hand sign, he could release a cloud of poison that spreads over a large area, affecting multiple enemies at once. The poison could cause various effects like slow paralysis or confusion.
Some music weapons might be able to heal. A harp - like weapon could play soothing melodies that accelerate the healing process of injured ninjas. This would be a great asset in the midst of battles or in the aftermath.
It could be the power to control others' emotions through singing. For example, the character could sing a sad song and make the enemies feel depressed and lose their will to fight.