Naruto could gain some really cool and unique abilities. For instance, he might be able to transform his body into different forms, like a phoenix made of chakra and magic energy. His speed could be enhanced to a superhuman level, not just by chakra - enhanced movement but also with the magic in his bloodline adding an extra boost. He could also develop the ability to create illusions that are not just based on chakra, but also on the magic within him. These illusions could be so powerful that they can deceive even the most experienced ninjas.
Naruto could have the power of solar - powered regeneration. The sun is often associated with life and renewal. So, if he gets injured, his wounds could heal faster with the energy of the sun. He may also be able to use the sun's energy to enhance his physical strength. He could run faster, jump higher, and hit harder as if he was charged up by the sun's power.
He could gain the ability to freeze Grimm. In RWBY, silver eyes have the power to harm and sometimes even stop Grimm in their tracks. Naruto's already impressive combat skills combined with this would make him a formidable force. He might also gain enhanced vision, like being able to see in the dark better or detect hidden threats more easily.
He might gain the ability to cut through any chakra - based defenses. Since the Master Sword is often depicted as a very powerful weapon, it could enhance Naruto's combat capabilities against tough opponents.
He could probably gain the ability to phase through objects. Just like wraiths are often depicted as being able to pass through solid things. This would give him an edge in combat and also in sneaking around.
The sword could give Naruto an advantage in close - quarter combat. Since Naruto is mainly known for his long - range attacks and his ability to transform into the Nine - Tails, having Zabuza's sword would force him to improve his close - combat skills. This could lead to him developing new evasion and counter - attack maneuvers, making him a more well - rounded fighter in the fanfiction.
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.
In some fanfictions, Naruto's black chakra might represent a new, darker side of his power. It could be related to some sort of ancient or forbidden energy. For example, it might give him the ability to access shadow - like techniques that are more in tune with the darker aspects of the ninja world, like stealth and silent assassination skills that normal Naruto doesn't typically use.
He might gain the ability to control wood - style jutsu more effectively. Since Hashirama was known for his powerful wood - style, Naruto could use it to create huge forests for defense or to trap enemies. For instance, he could grow trees instantly to block incoming attacks.
Naruto could gain the ability to manipulate fire in a unique way. Since the Kyuubi is associated with a certain element - like energy, it could translate into Naruto having a new form of elemental jutsu. He might also be able to heal faster. Kyuubi's chakra has shown to have regenerative properties, and with a bloodline, this could be enhanced in Naruto, allowing him to recover from injuries more quickly.
He could gain the ability to create large - scale wooden structures, like Hashirama did. For instance, he could build huge wooden golems to fight for him in battles.