It could be a result of some sort of experimental jutsu gone wrong. A mad scientist - like ninja was trying to create a new form of power and accidentally opened a portal to the digital world, bringing Black Agumon through. Naruto then has to deal with the consequences of this new arrival in his world.
Black Agumon could be a summon. Similar to how ninja summon animals, a character in the Naruto world discovers a way to summon Black Agumon. Its digital world powers would make it a very unique and powerful summon, changing the dynamics of battles.
In fanfiction, black fire bending can be seen as an exotic and powerful addition to the Naruto universe. It could be a result of some ancient and forgotten jutsu that has been rediscovered. For example, a hidden scroll might hold the secrets to black fire bending, and a curious ninja stumbles upon it. This new power then sets them apart from other ninjas, as black fire is often depicted as more dangerous and harder to control than regular fire in fanfiction.
In Naruto fanfiction, General Grievous could be from a far - off land that has a different type of chakra system. His appearance in the Naruto world could be due to a prophecy. His arrival could disrupt the balance of power. Naruto, being the protagonist, would see Grievous as a new challenge. Grievous' mechanical body might be resistant to some of the common ninjutsu in Naruto, which would force the characters to come up with new strategies to defeat him.
He could be a mentor figure. Given his immortality and vast combat experience from the Naruto world, he could teach the demigods new ways to fight and strategize.
Well, Goku Black could be a powerful antagonist in the RWBY fanfic. His combat skills and powers from the Dragon Ball universe would make him a formidable foe. He could have his own motives, like seeking power in the RWBY world or trying to impose his own sense of order. For example, his ability to use ki blasts could be a new and terrifying threat to the Huntsmen and Huntresses in RWBY.
Well, a character with the Black Angel Bloodline would bring a whole new dynamic to the Naruto universe in a fanfiction. Firstly, their appearance would be quite distinct. With features that might be a mix of angelic and dark elements, they would stand out. Their abilities would require them to develop new fighting styles. They could use their enhanced senses to detect traps or hidden enemies better than most ninja. In terms of relationships, they might befriend some of the more open - minded ninja, like Naruto himself who is known for accepting others regardless of their differences. However, they would also have to deal with the more traditional ninja who are skeptical of new and strange powers. They could also have a role in exploring new areas of the Naruto world that are difficult to reach for normal ninja, using their flight ability.
Well, Gojo could be a super powerful outsider. Maybe he comes to the Naruto world to study its unique chakra system. His Infinity ability would be a game - changer. He could use it to easily deflect most ninjutsu attacks. And his Six Eyes could help him detect hidden enemies or traps that normal shinobi might miss.
Another way Cerberus can fit into the Naruto world in fanfiction is as a spiritual guide. Maybe Naruto, during a near - death experience, enters a spiritual realm where he meets Cerberus. Cerberus then guides him on a journey to discover new powers within himself. This could be related to Naruto's connection with the nine - tailed fox and how Cerberus helps him better understand and control that power.
Well, Goku's extreme power level could make him a sort of legend in the Naruto fanfiction world. His Saiyan abilities like the Kamehameha and Super Saiyan forms would be completely foreign to the ninjas. He might be seen as an otherworldly being, and his presence could disrupt the power balance. Maybe he'd come to the Naruto world seeking a new challenge, or perhaps he was accidentally transported there.
Dark Naruto might be introduced as a threat in a Marvel fanfiction. His darker nature could lead him to target powerful artifacts in the Marvel Universe, thinking they could enhance his power. For example, he could go after the Infinity Stones. His combat skills and unorthodox powers would make him a formidable adversary for characters like the Avengers.
In fanfiction, Godzilla could be a protector of the Earth in a different way than the Avengers. The Avengers protect the people from threats, while Godzilla protects the balance of nature. So, when an alien race starts to mine the Earth's core in a way that disrupts the planet's stability, Godzilla appears. The Avengers at first misunderstand Godzilla's intentions. But as they see the real threat, they join forces with Godzilla. Godzilla's size and strength would be a huge asset against the technologically advanced alien invaders. The Avengers could also learn from Godzilla about the Earth's natural defenses.