Obviously, Villain Deku would be the main antagonist. Since he has turned to the dark side, his actions and motives would be in direct opposition to Jirou and the values of the hero world she represents.
It would be Villain Deku. He is the one with the evil intentions in this scenario. He could be causing chaos, harming people, and trying to bring down the hero system, all while having some sort of interaction with Jirou that puts her in a difficult position. For example, he might be blackmailing her or using her friends against her to make her do things his way.
One possible plot could be that Villain Deku, with his newfound dark powers and motives, targets Jirou as part of his grand scheme. Maybe he wants to use her unique quirk for his own evil ends. He might start by getting close to her, pretending to befriend her while slowly manipulating her into doing things that would benefit his villainous plans.
Another possibility is that a new villain emerges as a result of the time - travel or whatever mechanism brings future Deku to the present. This villain could be from a future where Deku's actions (or inactions) have led to a more dangerous world, and he travels back in time to stop Deku from making those mistakes again. However, his methods are extreme and dangerous, making him the antagonist of the story.
Well, I really liked 'The Unlikely Bond'. In this fanfic, Deku and Jirou start off as just classmates but gradually form a deep connection. It's interesting to see how they interact with each other's friends as well. And then there's 'Melodies and Mights'. This one is special because it combines Jirou's love for music with Deku's heroic journey. It gives a unique perspective on their relationship. If you like fanfics that focus on character growth within the relationship, these are great choices.
In a 'Villain Deku x Todoroki Fanfic', the characterizations would be key. Deku as a villain could have a new set of motives. For example, he might be disillusioned with the hero system and decide to take a different path. Todoroki, on the other hand, might be initially conflicted about his feelings for Villain Deku. Their relationship could be a complex mix of attraction and moral conflict. The fanfic could also explore how their combined powers might be used in different ways compared to their usual heroic uses. Maybe they form a sort of anti - hero duo that challenges the traditional notions of good and evil in the My Hero Academia universe.
It could be a villain who is obsessed with creating chaos through strange transformations. This villain might have targeted Deku specifically because of his status as a rising hero. They want to see how Deku will cope with being turned into a girl and use it to undermine his confidence and hero image.
Perhaps Mister Sinister. He has always been interested in mutants like Wolverine and could see Deadpool as a useful pawn or a nuisance in his grand schemes. He might try to manipulate the situation to his advantage, causing Deadpool and Wolverine to join forces against him.
It could be a powerful, corrupted Pokemon trainer who wants to capture Cosmog and use its unique powers for his own evil purposes. He might see Cosmog as a key to unlocking ultimate power and doesn't care about the harm it could cause to Cosmog or the Pokemon world. He could also be in league with some dark - type Pokemon that can create obstacles for Marshadow and Cosmog.
Mr. Burns could potentially be the antagonist. He might see the combination of Spongebob and Homer as a threat to his power in Springfield. He could try to separate them and cause trouble for them. Maybe he wants to take over Bikini Bottom as well, seeing it as a new source of resources or power. He starts by spreading rumors about Spongebob and Homer in both their hometowns to turn the locals against them.
It could be Darth Sidious. Since he is always looking for power and control in the Star Wars universe, he might see Naruto as a threat to his plans when Naruto arrives in Padme's world. He would try to eliminate Naruto and use Padme as a pawn in his political games.
It could be Ra's al Ghul. He's always interested in things that can give him an edge in his quest for immortality. If he discovers the power of chakra in the Naruto world, he might try to take over both worlds. His skills as a master assassin and his vast resources would pose a huge threat to Naruto and Batman, who would have to put aside their differences to defeat him.