Maybe through some kind of special training or a unique ritual. It could be that he discovers a hidden method within the ancient scrolls of his village.
There are several possible ways. It could be that Naruto has a near - death experience which causes a deeper connection with the Kyuubi, allowing him to access more of its power. Another possibility is that a powerful sage or shinobi helps him unlock the full potential of the Kyuubi's powers. Or perhaps there is a mysterious artifact that when combined with Naruto's own chakra, enables him to gain the Kyuubi's powers.
In the fanfiction, Naruto might gain the Kyuubi's powers due to a series of events. For instance, he could be exposed to a strange energy source that reacts with the Kyuubi inside him. This reaction could break down the barriers that were previously preventing him from fully accessing the Kyuubi's power. Or it could be that Naruto's strong will and his constant struggle to protect his friends and village impresses an ancient spirit that then grants him the ability to fully harness the Kyuubi's powers. Also, there could be a genetic factor that is suddenly activated, which is related to the Uzumaki clan's connection to the tailed beasts, allowing Naruto to gain the Kyuubi's powers.
Well, in the 'naruto gains kyuubi's powers fanfiction', Naruto might face new challenges as well as new opportunities. On one hand, he would be a target for those who fear his new - found power. On the other hand, he could use this power to protect his village better. He might also develop new techniques that are a combination of his own skills and the Kyuubi's power, like a new type of shadow clone jutsu infused with the Kyuubi's chakra.
In fanfiction, when Naruto has the full Kyuubi, he often gains enhanced physical strength. He can deliver more powerful punches and kicks. His speed also increases significantly, allowing him to move so fast that his opponents can barely keep up.
At first, Naruto might see the Kyuubi as just a source of power. But as he starts to protect it, he realizes it's more of a partner. Their relationship evolves from a simple user - power source dynamic to a deep - seated friendship. He begins to understand the Kyuubi's emotions and past, which further cements their bond.
In this fanfiction, their relationship becomes more of a partnership. Instead of Naruto just using the Kyuubi's power, they work together as brothers. Naruto might start to understand the Kyuubi's feelings and motives better.
One possible power is the ability to see through illusions more easily. Since the Kyuubi is a powerful entity, a doujutsu from it could give Naruto an edge against genjutsu users. It could break through complex illusions that would otherwise trap him.
He might gain it through a series of trials. Maybe he has to face his deepest fears in some kind of mental realm where the Kyuubi resides.
Kyuubi's comfort can make Naruto more confident. Knowing that he has Kyuubi's support, he may take on more difficult missions without hesitation. For instance, if he's facing a powerful enemy, he'll remember Kyuubi's words of encouragement and stand his ground firmly.
Well, Kyuubi might influence Naruto's growth by providing him with a sense of power from within. Since Kyuubi is like a mother figure, she could be a source of emotional support too. Naruto may draw strength from their connection, both in tough battles and in dealing with his own insecurities.
Naruto's powers, especially with the Kyuubi, would be a significant addition to the Justice League's capabilities. His ability to harness the Kyuubi's chakra gives him superhuman strength, speed, and regeneration. In a fanfiction, this could be used to complement the powers of the Justice League members. For example, he could use his speed to assist The Flash in a race against time, or his strength to back up Superman in a tough fight.
Well, generally his power would increase significantly. He'd likely have enhanced physical abilities like strength, speed, and stamina. His chakra reserves would also be much larger, allowing him to perform more powerful jutsus for a longer period without getting exhausted.