If Naruto turns into a super villain in fanfiction, perhaps his transformation is due to some dark influence. It could be a powerful and evil chakra that corrupts him. He would then start to use his jutsu in cruel and unexpected ways. For example, instead of using the Rasengan to protect, he might use it to destroy important buildings or harm innocent people. His friends would be shocked and try to bring him back to the good side, but he would be too far gone. This could lead to some really intense battles and emotional confrontations between him and his former comrades.
If Naruto becomes a villain in the DC crossover, he might set his sights on Superman. Naruto could believe that Superman's power is a threat to the balance he wants to create. He would use his ninjutsu to try and take on Superman. For example, he could use his Rasengan, a powerful spinning ball of chakra. However, Superman's strength and heat vision would be tough for Naruto to overcome. This would lead to an epic battle between the two, with the other DC heroes and Naruto's possible allies getting involved as well.
If Naruto loses all his friends in a tragic way, perhaps in a battle against an overwhelming enemy that the village could have prevented. His grief could turn into hatred towards the shinobi system. He may decide that the existing order is what causes all this pain and destruction, and then he starts to work against it, becoming a villain in the eyes of the village.
It could also be due to a strange prophecy. In both the Naruto and Dragon Ball worlds, prophecies play a big role. There could be a prophecy that a hero from one world would gain the powers of the other to defeat a common enemy. Naruto, being the hero of his world, is chosen and through some spiritual or magical transformation, he becomes a Super Saiyan.
In a fanfiction where Naruto becomes Obito, it could be due to an outside force. For example, an ancient evil entity that wants to disrupt the balance of the ninja world. It targets Naruto and messes with his mind, implanting Obito - like thoughts and desires. Naruto would then struggle between his original self and this new, Obito - influenced persona, leading to many internal and external conflicts within the story.
Well, in this kind of fanfiction, it might happen through some sort of cross - dimensional magic. Maybe there's a rift between the Naruto universe and the Dragon Ball universe that allows Naruto to gain the power of a Super Saiyan God. It could also be the result of a powerful wish made by a god - like character who decides to give Naruto this new form for the sake of saving his own universe or something like that.
It could be Kabuto. Since he has a lot of knowledge about medical ninjutsu and experiments, he might see Naruto's robotic form as a threat or something he can use for his own evil purposes. He could try to control Naruto's robotic functions and turn him against his friends.
One possible plot could be that Adrien discovers some dark secrets about his family or the superhero world. Maybe he feels betrayed by Ladybug or the other heroes. This disillusionment could push him towards the dark side. For example, if he finds out that his father was a victim of the so - called 'good' superheroes' actions in the past, he might want revenge.
In a 'Danny Phantom becomes villain' fanfiction, it could be that Danny has a major falling out with his friends and family. He feels betrayed and abandoned. This emotional turmoil causes him to snap and turn to the dark side. He then uses his ghost powers to take revenge on those who he thinks have wronged him, becoming a feared villain in the process.
The plot could start with a dark spell or some kind of mind - altering event that turns Harry into the Joker - like figure. Then, he might start attacking the very people he once protected, like his classmates at Hogwarts. This would create a lot of chaos within the school walls. The teachers would be at a loss on how to deal with him, as his magic combined with the Joker's unpredictable nature would be a very difficult combination to handle.
If Naruto becomes super IQ in the fanfiction, he could potentially become a master of multiple fields. He could be an excellent diplomat, using his intelligence to negotiate treaties and alliances. He would also be a great inventor, creating new tools and weapons for the ninja world. His friends might be a bit in awe of his newfound intelligence at first, but they would soon realize that he's still the same Naruto at heart, just with a much sharper mind.