Well, it could be that Naruto's own chakra nature evolves in an extreme way. His Uzumaki bloodline might have some hidden potential that, when combined with his experiences and growth as a ninja, mutates into the ability to control black holes. For example, his chakra, which is already very powerful, could reach a new level where it can warp space to form black holes.
One possibility in this fanfiction is that Naruto is exposed to a powerful and otherworldly energy source. This energy could be from a different dimension or a long - forgotten relic. When he comes into contact with it, his body absorbs and adapts to it in a way that allows him to control black fire. This new power also comes with its own set of challenges and responsibilities for Naruto as he tries to integrate it into his existing set of skills.
He might face powerful rogue ninjas. These ninjas could be after the power of the black fire for themselves. They would see Naruto as a threat and try to take him down to get the power.
Foxes are quite common. Given Naruto's connection with the Nine - Tailed Fox, it makes sense that in fanfiction, he might be able to control other foxes. They can be used for scouting or as distractions in battles.
One interesting concept could be Naruto using his chakra in a way that interacts with the extreme forces of a black hole. Maybe he could develop a new jutsu inspired by the black hole's gravitational pull.
Obviously Naruto himself is often involved. He's the main character of the original series, so in fanfiction about Naruto and black holes, he would be at the center of the story. Sasuke might also be involved as his rival and sometimes ally.
One idea could be that Naruto gains some new power or jutsu that allows him to control Anko's actions in a non - malicious way. Maybe it's a form of genjutsu that he accidentally uses on her during a training session, and then has to figure out how to reverse it.
In a 'Naruto controls snow' fanfiction, it could be that Naruto is cursed by an enemy. As a result, his chakra is somehow linked to snow. At first, he struggles to control this new power, but with the help of his friends and some intense training, he masters the ability to control snow and turns it into a powerful weapon against his foes.
In the fanfiction where Naruto controls the Tesseract, it could change the power dynamics completely. Naruto is already a powerful ninja, but with the Tesseract, he could become near - invincible. He might be able to use the Tesseract's space - bending abilities to surprise his enemies in battle. For example, he could suddenly appear right in front of them or make them disappear into another dimension. Also, he could potentially use it to heal wounds or revive the dead, if he discovers all of its powers, which would make him not just a hero in his own world but maybe in the entire multiverse of the fanfiction.
In a more comical fanfiction, Naruto might accidentally control Anko's eating habits. Let's say he uses a new jutsu that backfires and instead of affecting an enemy, it affects Anko. So, every time Naruto thinks about a certain food, Anko starts craving it and can't resist. This would lead to some really funny situations as Anko tries to fight against this strange control while Naruto is desperately trying to figure out how to undo the jutsu.