In certain fanfictions, Naruto's immunity to poison might be a result of his training with a hermit or some ancient sage. They could have taught him special techniques or given him elixirs that made his body resistant to poison over time. For example, he might have had to endure harsh trials in a poisonous environment, and his body gradually adapted to it, leading to complete immunity.
It could be due to a curse or a blessing. Maybe in the process of traveling between the Naruto universe and the DxD universe, some sort of magical energy latched onto Naruto and made him immortal. For example, it could be related to the Sacred Gears in the DxD world. A Sacred Gear could have malfunctioned or had an unexpected reaction when Naruto entered the world, granting him immortality.
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.
Well, in some fanfics, Naruto's death could be like a sacrifice that unlocks some hidden or ancient power. Maybe his death releases a special chakra that was locked within him, and this chakra then transforms into new, more powerful abilities. It could also be that his death is part of a prophecy, and upon his death, a deity or some other powerful entity grants him powers as a reward for his selfless act of sacrificing himself.
In some Naruto fanfictions, Naruto's chaos powers might be shown as a wild and unpredictable energy source. It could make his jutsu more erratic but extremely powerful. For example, his Rasengan might become a chaotic, swirling mass of energy that can break through even the strongest defenses with ease.
In a Naruto - Superman powers fanfic, perhaps Naruto gets some of Superman's powers accidentally. He struggles to control the new powers like heat vision and super speed along with his own ninja abilities. Superman, on the other hand, discovers he can use chakra in a limited way and they both have to figure out how to use these new combinations of powers effectively.
The 'godlike' powers could be integrated as an evolution of Naruto's existing chakra powers. Maybe he discovers a new form of chakra that gives him 'godlike' abilities like the ability to create or destroy matter at will. The 'flash' powers could be added to his movement skills, like he can move at super - speed by channeling this new form of chakra.
Naruto's character development would likely be a wild ride. Bill Cipher's powers are extremely powerful and otherworldly. Naruto could start to see the world in a different light. He may become more detached from the normal ninja way as he explores the limits of his new powers. This could lead to some conflicts with his friends, as they might not understand the new Naruto who has these strange and dangerous powers. He might also gain a new sense of arrogance at first, thinking he can solve all problems with his newfound abilities, but then have to learn humility through some tough experiences.
Well, Naruto's powers are mainly centered around his chakra control and the unique jutsus he has learned. In High School DxD, the powers are often related to sacred gears or devil abilities. If Naruto were to enter this world, his chakra might be seen as a new type of energy source. His ability to summon toads could be a huge advantage in certain situations. For instance, if he were in a fight against a powerful devil, he could summon Gamabunta, who could cause chaos and distraction. On the other hand, the devils' powers of illusion and teleportation could pose new challenges to Naruto, forcing him to adapt his combat style. His Rasenshuriken, which is a highly destructive jutsu, might be able to break through some of the stronger defenses in the DxD world.