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.
In this fanfiction, the prototype powers could be something like an early form of chakra manipulation that is different from the traditional Naruto world's chakra use. It might be more raw and unrefined, with the potential for new and unique jutsu.
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.
One possible new power could be the power to absorb and use any chakra, regardless of its origin. This would make him extremely powerful as he could draw from the chakra of his enemies or even from natural sources like the sun or the earth. He could also gain the power of telepathy on a large scale, being able to communicate with anyone in the world or even across different dimensions. Additionally, he might obtain the power of instant teleportation to any place he desires, which would give him a huge advantage in battles and travels.
Well, he could use Freddy's power of entering people's dreams to fight his opponents in the dream realm. Maybe he would trap his enemies in a never - ending nightmare, causing them mental distress and weakening their will to fight in the real world.
His hidden powers might be the ability to control all five basic nature types without any training. In the original story, Naruto had to work hard to master different nature types. But in a fanfiction, it could be that he has this innate ability which was hidden all along. This would make him extremely powerful and feared by his enemies.
In some fanfictions, Naruto's multiverse powers are portrayed as a way to bring together different versions of himself. Each version has its own set of skills and knowledge. By merging these aspects, he becomes an even more powerful and complex character, able to solve problems that were previously insurmountable.
Well, it could be that there's a plot device like an ancient artifact. This artifact might have the power to draw out and combine the Akatsuki powers. The Akatsuki members themselves might be tricked into contributing their powers to this artifact, and then the protagonist or an antagonist gets hold of it and gains all those combined powers. It's a common trope in fanfiction to use such magical or mysterious items to bring about extraordinary power combinations.
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.