Perhaps in the fanfic, Izuku is possessed by some dark entity which changes his behavior and makes him do terrifying things, causing fear among his friends and foes alike. For example, his actions might be unpredictable and seem dangerous to those who know him. He could start speaking in strange voices or have sudden outbursts of strange powers that startle everyone. This would lead to a situation where everyone is scared of him, and the story could revolve around how he tries to get rid of this possession or control this new and scary side of himself.
One possible plot could be that Izuku has been through a really traumatic experience that has changed him. He starts to isolate himself and when people try to approach him, he acts in a way that scares them off. He could have developed some sort of survival - based aggression. For instance, if he was trapped in a really dangerous situation where he had to fight to survive, he might now react harshly to any sudden movements or people getting too close to him, which would scare his friends and classmates.
His classmates like Bakugo could be a main character. Bakugo might be one of the first to notice the strange changes in Izuku and could be both scared and angry about it. He might try to confront Izuku, which could lead to some intense scenes.
His friends from U.A. High School such as Bakugo, Ochaco, and Iida would also likely be main characters. Since Izuku is scaring everyone, they would be the ones experiencing the fear and trying to figure out what has happened to him. Their reactions and attempts to deal with the situation would be important parts of the story.
The plot might involve Naruto entering the Kyuubi's mental space in a new and more powerful way. He could bring with him some sort of 'light' or positive energy that is anathema to the Kyuubi. This positive energy could represent all the good things in Naruto's life, like his friends, his dreams, and his love for the village. The Kyuubi, being a creature of darkness and power, is scared by this pure and positive force. Or maybe Naruto discovers a hidden weakness of the Kyuubi through his own growth and understanding. He then exploits this weakness, causing the Kyuubi to be afraid of what Naruto could do to it.
Well, it could start with Izuku waking up as a girl one morning with no explanation. He (now she) tries to keep it a secret at first but then realizes it's not possible. The story could focus on how Izuku's relationships change. For example, Bakugo might initially tease her more harshly but then come to respect her in a different way. And Midoriya could find that she has a different approach to using her quirk as a girl, maybe being more strategic or having different physical capabilities that she has to learn to master.
In the 'izuku becomes a girl fanfic', perhaps the plot involves Izuku adjusting to her new physical form. She might struggle with things like different clothing styles and how society views girls in the hero world. Also, her relationship with other characters could change. Her friendship with Ochaco might become more complex as they now have more in common as girls. And Bakugo might react in a really interesting way, either being more protective or even more competitive towards her.
Another aspect could be the general discrimination in the BNHA society. The fact that society is so focused on quirks and those without are often looked down upon might have made Izuku bitter. He could be hating everyone because he sees the society as a whole being corrupt and unkind, not just the individuals who directly wronged him.
It could be that Ash and Pikachu are trying to play a prank on Gary. Pikachu jumps out and scares Gary who is with his Electrivire. Then Gary might get angry at first but then they end up having a friendly battle or competition after the initial scare.
In the 'izuku secretly dating a girl fanfic', the plot could be that the girl is actually from a rival school. Izuku is initially hesitant about the relationship because of the potential conflict of interest. But their feelings for each other are too strong. They date secretly, and there could be some close calls where they almost get caught by classmates or teachers from either school.
In the fanfiction, Naruto could have been possessed by some dark force. This dark - Naruto then starts to act in a very unpredictable and scary way, like randomly attacking people with great force. He might also have a really creepy look in his eyes, which scares everyone around him. The villagers who once knew Naruto as a friendly ninja would be shocked and terrified.