One possible story could be that Uraraka turns villain due to being corrupted by power. Maybe she starts to use her quirk for self - gain, like stealing valuable items and floating them away. She could form an alliance with other villains, believing that the hero society has failed her.
Well, a 'My Hero Academia Villain Killer' fanfiction can be really exciting. It often explores the darker side of the MHA universe. Some fanfictions in this genre might focus on a new character who has a vendetta against villains. They could introduce unique powers and moral dilemmas. For example, the character might struggle with the line between justice and revenge while taking on the most heinous villains in the series. It adds a whole new layer to the already rich MHA world.
To create a compelling villain in My Hero Academia OC fanfiction, first, give them a unique quirk. It should be powerful yet have some limitations. For example, a quirk that can control time but only for a few seconds at a time. Second, develop their backstory. Maybe they were once a hero-hopeful but were betrayed by the hero society. This would give them a reason to turn to villainy. Finally, give them a distinct personality. They could be charming yet extremely cruel, like the Joker from Batman.
Dabi could also be a main villain in such a fanfiction. His mysterious past and his powerful fire - based quirk would make for an exciting addition to UA. He could go to UA to spy on the heroes - in - training, but end up getting involved in their lives more than he expected. His cynical view of the hero society would clash with the positive environment at UA, and he might even start to question his own motives while there.
One popular archetype is the power - hungry villain. They crave more and more power, like All For One in the original series. Another is the vengeful villain. They have a grudge against someone or the whole hero society and will stop at nothing to get their revenge. There's also the mad scientist type. They conduct unethical experiments, often creating dangerous things in the pursuit of knowledge or power.
Well, if Naruto with the Rinnegan were a villain in My Hero Academia fanfiction, it would be a huge shift in the power balance. His Rinnegan gives him access to a variety of powerful abilities. He could use the Asura Path to create mechanical weapons and attack the heroes. The heroes would need to band together and come up with new strategies. His motives as a villain could also be complex, perhaps influenced by his experiences in the Naruto world. This would add depth to the fanfic and make it a very interesting read as the heroes struggle to understand and defeat him.
Tony Stark as a villain would bring a whole new level of complexity. His backstory could be that he was rejected by the hero society in the My Hero Academia world and decided to take revenge. He'd use his skills to create a network of minions and build a secret lair. His battles with the heroes would be epic, as he'd constantly upgrade his technology to outwit them.
It could be that Naruto was influenced by a powerful villain in the My Hero Academia world. This villain could have promised Naruto something he always wanted, like true acceptance or power beyond his wildest dreams. So, Naruto turned to the dark side. In a fanfiction, this could lead to really interesting battles and character interactions, especially with the heroes of My Hero Academia.
She could be influenced by a villain's ideology. For instance, if she was convinced that the current hero system is unfair and only benefits the strong. She might start to target heroes who she thinks abuse their power. Using her quirk, she'd be a formidable foe as she can manipulate gravity around objects and people.
Toga is also very popular. Her quirk and her somewhat unhinged personality give a lot of room for creativity. Fanfictions can explore her view of the world and her relationships with other villains in more detail. Maybe even a story about her trying to find a 'normal' life despite her villainous nature.