He might gain the ability to cut through almost anything. The black lightsaber, being a powerful weapon, could enhance Naruto's already formidable combat skills. He could use it to slice through strong jutsu defenses with ease, something that normal kunai or shuriken couldn't do.
The black lightsaber could also give Naruto the power of flight or levitation in some fanfiction concepts. By manipulating the energy of the lightsaber, he can use it to propel himself into the air, allowing for new combat strategies and the ability to reach enemies in hard - to - reach places, adding a whole new dimension to his fighting capabilities.
With the black lightsaber, Naruto could potentially have new ways of channeling his chakra. For example, he could infuse the lightsaber with his own unique chakra nature, like wind - style chakra, to create a more powerful and deadly weapon. This could give him an edge in battles against opponents who are used to his regular combat style.
Perhaps it's given to him by a mysterious figure. This figure could be from another dimension or a very old ninja who has been in hiding for a long time. They recognize Naruto's potential and power, and decide that he is the one worthy of wielding the black lightsaber. This could be part of a bigger plan to help Naruto defeat a new and powerful enemy that neither the ninja world nor the normal means of combat can handle.
He probably gains the ability to control the chakra of the Kyuubi more freely. This could mean enhanced strength, speed, and reflexes. He might also be able to use some of the Kyuubi's unique jutsu like the Tailed Beast Ball. His healing factor could be improved as well, allowing him to recover from injuries faster.
In such fanfiction, Naruto may gain the ability to heal himself more quickly. Since the tailed beasts possess a great amount of life - force energy, Naruto could tap into that to accelerate his own healing process. His mental abilities could also be enhanced. He would be able to withstand greater mental pressure and use the tailed beasts' collective wisdom to make better decisions in battles. Moreover, he could potentially create a chakra shield that is extremely strong, using the combined chakra of all the tailed beasts to protect himself and his allies.
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.
Well, he might gain the ability to create spider - like webs for trapping enemies. It could be a really cool addition to his ninja skills, allowing him to set up traps quickly and immobilize opponents.
He might gain a deeper understanding and control over the Six Paths powers. Maybe he can access different levels of chakra that were previously unknown. For example, he could have a more direct connection to the natural energy around him, allowing for more powerful and precise jutsu.
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.
Naruto might gain the power of shikai and bankai. Shikai is the first release of a zanpakuto's power, and bankai is the ultimate form. Naruto, with his determination and strong will, would strive to master these forms. Shikai could give him new attacks and defenses, while bankai could make him an extremely powerful force. For example, his bankai could combine his ninjutsu skills with the zanpakuto's power in a unique and devastating way.
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.