It could be Sasuke. If Sasuke continues to pursue his own vendetta in a more extreme way and Naruto's attempts to bring him back fail, Sasuke could become the main antagonist. Their rivalry could turn into a full - blown battle for power and ideology, with Naruto being pushed to his limits and potentially turning dark in the process of trying to stop Sasuke.
Another plot could involve Naruto being influenced by a powerful and evil entity. This entity could be a new, powerful villain who manages to corrupt Naruto's mind. It could start with small suggestions that gradually change Naruto's perspective. As he becomes more and more influenced, his actions start to become more self - centered and less about protecting the village and more about achieving his own, perhaps twisted, goals.
Well, in fanfiction, the Dark Phoenix turning good could be a complex and multi - faceted process. It might start with her being exposed to a different ideology, perhaps one of peace and unity. This could make her question her current state of being. Additionally, her powers could be used in a more positive way. For instance, instead of causing destruction, she could use them to heal or protect. The author might also explore her internal struggle between the dark and the good within her, and how she finally overcomes the dark side. This could involve her facing her past traumas and using them as a stepping stone towards becoming a better version of herself.
One common element in the Sonic turns into Dark Sonic fanfiction could be the role of antagonists. They might have a hand in triggering this transformation. For example, by manipulating Sonic's emotions or by using some sort of dark magic. Once Sonic becomes Dark Sonic, his relationships with his friends like Tails and Knuckles also change. His friends may try to bring him back to his normal self, which can create a lot of drama and action in the fanfiction.
Obviously, Jack Frost is the main character as the story is about his transformation into a dark version of himself. But there could also be other characters from the original Jack Frost lore, such as the Guardians like North, Bunny, and Tooth. They would likely play important roles, either trying to save Jack from his dark path or being forced to oppose him if he becomes too dangerous.
Another plotline could involve a betrayal within the group that causes Elsword to turn dark. For example, if he discovers that one of his closest allies has been secretly working against him all along, the shock and anger could drive him to the dark side. He might then start to seek revenge on not only the betrayer but also those he perceives as being part of the conspiracy.
It could be an ancient dark wizard who has been manipulating events from the shadows. This wizard might see Elsword as a powerful tool and has been slowly influencing him to turn dark. The wizard could be using dark magic to cloud Elsword's mind and make him more susceptible to negative emotions.
Perhaps in the fanfiction, the Dark Phoenix is exposed to a new world or dimension where good prevails. This new environment makes her reevaluate her actions and she decides to turn good. For example, she might see how the inhabitants of this place live in harmony and without the chaos she has been causing. This contrast could be the push she needs to change. She could also start to form relationships with the positive characters in this new world, which further encourages her transformation.
One possibility is that she becomes disillusioned with the Rebellion. Maybe she discovers some dark secrets within the Alliance that make her question their motives. For example, if she finds out that the Rebellion has been using unethical tactics in the fight against the Empire. This could push her towards the dark side as she feels betrayed by those she once trusted.
I haven't heard of a fanfiction where Kyuubi turns Naruto into a futa. Naruto fanfiction often focuses on things like character development, new adventures, or different power - up scenarios. This concept is rather unorthodox and might not be as common as more mainstream fanfiction ideas. It's possible that it exists in some corner of the vast fanfiction universe, but it would take some serious searching to find it.