It could be that he is influenced by a powerful and evil force. For instance, a new super - villain uses mind - control or some form of manipulation on Batman, turning him against his former allies and making him commit villainous acts.
He might be motivated by a sense of futility. After years of fighting crime and seeing no real end to it, he could think that the only way to bring about change is to control the criminal underworld himself. So, he becomes a villain to enforce his own version of order.
One possibility could be that Batman, after facing one too many betrayals in Gotham, decides to take a different path. Maybe the constant struggle against crime with little real change in the city makes him cynical. He could start using his skills and resources to control the underworld instead of fighting it. For example, he might start dictating which criminals can operate and where, creating a new order that is still dark but under his command.
The Joker could be a great villain. Imagine if the Joker - like character emerged in the My Hero Academia world. Izuku, with his new Batman - inspired skills, would have to use his wits to outsmart this chaotic and unpredictable foe.
One possible motive could be a deep sense of betrayal. If he discovers that the village leaders have been using him all along and that his friends were in on it, he might snap and turn to killing those he feels have wronged him. Another motive could be the influence of a dark power. Maybe he comes across some ancient evil that corrupts his mind and turns him into a killer.
Ra's al Ghul could be a great choice. He might see the daughter as a potential heir or a threat to his plans for Gotham. He could try to manipulate her or use her against Batman. This would force Batman to not only protect his daughter but also try to make her understand the true nature of Ra's al Ghul.
Maybe Red Hood believes that Batman's methods are no longer effective. He could think that by killing Batman, he can take over Gotham and impose his own brand of justice, which might be more brutal but in his eyes more efficient. This would be a great plot point in fanfiction to explore different concepts of justice.
In this fanfiction, Izuku could be inspired by Batman's lack of superpowers but still being a formidable hero. He may start training in more martial arts and stealth techniques, similar to Batman. And he could even create his own set of gadgets, like grappling hooks and smoke bombs, to aid him in combat and rescue missions.
His confidence could turn into over - confidence bordering on arrogance. As a villain, he might think he is above everyone else and that his way is the only right way. He would no longer listen to advice or consider the consequences of his actions on others, believing that he can handle any situation on his own, no matter how dangerous or unethical.
One possible storyline could be that Jason Todd, after his resurrection, starts to see Batman in a new light. Maybe he begins to study Batman's methods more closely, realizing the good he was trying to do all along. He could start collecting Batman memorabilia and secretly helping Batman from the shadows, all while trying to come to terms with his own complex relationship with the Dark Knight.
Another option could be a new villain created specifically for the fanfic. This villain could be someone who has a vendetta against the Wayne family because of something Batman's dad did in the past. He could be a master of disguise and manipulation, making it difficult for Batman and his dad to figure out who he really is.
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.