Perhaps an unknown entity that has possessed the doll. This entity could come from another dimension where evil rules. It uses the doll as a form to enter the My Hero Academia world and plans to take over. It starts by causing strange and unexplainable events, and as the heroes try to figure out what's going on, it gets stronger and more menacing.
Well, it could be a doll that has the power to control the minds of the characters. Imagine a doll that makes the heroes act against their will, like making Deku use his power to harm his friends instead of save them. That would be really terrifying.
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.
It could be All For One. He might see the 'Villain Class 1A' as a useful tool to further his own evil plans. He could manipulate them into doing his bidding, which would put them in direct conflict with the heroes, including Class 1A.
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 could be an ally. Since she has her own complex relationship with the idea of heroes and villains, she might find Deku's transformation interesting. She could provide some of her unique skills, like shape - shifting, to help Deku in his villainous plans. Additionally, Twice might also join them because of his loyalty to Toga and his own sense of being an outcast in the hero world.
It could be Deku's son or daughter. They would inherit some of his amazing quirk, but also face the pressure of living up to their father's legacy. They would have to carve their own path in the hero world.
Perhaps an all - powerful villain from another dimension invades. This villain has the ability to control the minds of those with quirks and uses them to fight against the remaining heroes. He wants to take over the 'My Hero Academia' world and remake it in his own image. His powers are so strong that it takes all of the heroes' combined efforts just to try and figure out a way to stop him.
Bakugo could also make an interesting main character. His explosive personality combined with a gamer ability would be really cool. He could be super competitive in the 'gamer' aspects of the story, like ranking up in a hero - ranking system that works like a game. And he would be all about proving he's the best, not just in real - life heroics but also in the game - like challenges.
A group of villains who specialize in causing chaos during disasters could also be the main antagonists. They would constantly try to undermine Backdraft's efforts to keep the city safe during fires and other emergencies.