Maybe he gets the power of elemental fusion. He could combine fire and water to create a new, more powerful element. This would make his jutsu more unpredictable and stronger. For example, he could use it to break through tough defenses that normal jutsu can't penetrate.
One possibility could be the power to control time. In fanfiction, this could be used in various ways, like going back to prevent tragedies or to gain more time to train. It would give Naruto a huge advantage in battles and in his quest to protect the village.
Another new power could be the ability to communicate with all living things. This would allow him to understand the needs and motives of animals and plants. In a battle, he could call upon the help of animals, or use plants to his advantage, like creating barriers or traps.
Well, perhaps the new arm comes from some kind of ancient and mysterious source. It could be an arm that was once part of a powerful ninja from the past. This new arm could have a mind of its own, at first causing Naruto some trouble as he tries to control it, but eventually they work in harmony and it gives Naruto an edge in battles against tough enemies.
In this fanfiction, Naruto might learn new and unique jutsus. His new sensei could have a different teaching style, perhaps more strict or more creative, which would greatly impact Naruto's growth as a ninja.
He could have a 'Solar Sage Mode'. In this form, his body would be surrounded by a bright, sun - like aura. His physical abilities such as strength, speed, and agility would be enhanced to an extreme level. His attacks would also carry the power of the sun, like solar - powered punches or kicks that could cause massive damage.
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.
Maybe Orochimaru. Despite being a villain in the past, he is a master of forbidden jutsus and experiments. He could see Naruto as an interesting subject and give him a new arm with some dark powers, which Naruto would then have to learn to control for the greater good.
In the fanfiction, Naruto's new sensei could be a mysterious figure who has ties to the Sage of Six Paths. This person might have been waiting for the right time to appear and train Naruto. They could possess knowledge about the origin of chakra and how to fully unlock Naruto's power. It could also be someone who was once an enemy but has now changed their ways and wants to use their skills to train Naruto for the greater good of the ninja world.
They could be a sleek black ninja outfit with red accents. Maybe it has a high - collared jacket for a cool look and some new ninja sandals with better grip.
Naruto would adapt to Obito's Sharingan by relying on his strong will and his existing combat experience. He's faced many challenges before, and this would be just another one. He'd probably train with Kakashi or Sasuke to understand the Sharingan better. Since he has such a large amount of chakra, he might be able to use the Sharingan's power more frequently than others. He would also try to incorporate it into his existing fighting style, for example, using it to enhance his Rasengan or his taijutsu.
The new clothes could be a stylish ninja dress. It might have a bright color like cherry red, with a short skirt for easy movement during battles. There could also be some delicate lace details on the sleeves and hem.
He could potentially gain an enhanced understanding of wood - style techniques. Since Hashirama was a master of it, Naruto might learn how to create more complex and powerful wood constructs, like huge forests for both offense and defense.