Well, one thing that could happen is that Class 1a would be in for a huge shock. They've known Deku as a hero - in - training. Meeting him as a villain would completely shatter their perception. They might initially try to reason with him, thinking there's some misunderstanding. Maybe Bakugo would be the most conflicted, given their long - standing relationship.
Bakugo would likely be furious. He'd feel betrayed and would probably be the first to attack Deku, yelling at him about how he's thrown away everything they've worked for. Uraraka might be more empathetic. She'd try to understand what drove Deku to become a villain and hope there's a way to bring him back to the side of good.
One possible plot could be that Villain Deku is on a mission to undermine the hero society and crosses paths with Hero Deku. They engage in a fierce battle where Villain Deku taunts Hero Deku about his ideals. In the end, Hero Deku manages to get through to a small part of Villain Deku's conscience, leaving him with some doubts about his villainous path.
Villain Deku might start to question his villainous ways. Meeting Hero Deku could make him see the good in heroism. Hero Deku, on the other hand, could become more empathetic towards those who turn to villainy. He might realize that there are reasons behind it and not all villains are just 'evil' for the sake of it.
A possible plot is that Villain Deku has a plan to destroy a major hero institution, and Hero Deku has to stop him. But when they meet, Hero Deku realizes that Villain Deku was once like him, full of hope. This makes Hero Deku hesitant to fight at full force. Or perhaps Villain Deku has kidnapped someone close to Hero Deku, and during the rescue mission, Hero Deku tries to convince Villain Deku to change his ways. Also, they could be from alternate universes where the roles are reversed, and they are forced to interact and learn from each other.
For Hero Deku, his development could be shown by how he reacts to seeing his dark self. He might start to question his own beliefs and actions. For Villain Deku, his development could be in the form of a possible redemption arc. Maybe he starts to see the error of his ways through his interactions with Hero Deku. For example, if Hero Deku shows him kindness or understanding that he never received before.
Maybe in this fanfiction, Class 1 - A is on a training mission when they accidentally stumble into a time - warp and meet Future Deku. Future Deku could be a super - powerful hero, and he starts to train them in advanced techniques that they haven't learned yet. He might also share some of his future experiences with them, like the battles he has won and the villains he has faced, which would inspire Class 1 - A to strive harder in their hero training.
I think Uraraka would be very excited. She has always been a big supporter of Deku. Meeting Future Deku would give her a chance to see how far he has come, and she might also get some inspiration for her own hero journey. Plus, she's a friendly and emotional character, so she'd likely be thrilled at the opportunity.
You can start by looking on fanfiction platforms like Archive of Our Own (AO3). It has a large collection of My Hero Academia fanfictions, and you're likely to find some great 'class 1a meets manga deku' ones there.
There's also 'When Class 1a Meets Manga Deku'. This fanfiction focuses on how the students of class 1a react to a Deku who has all the powers and abilities as depicted in the manga. It shows their initial shock, followed by their attempts to work together with this new version of Deku. The story also delves into the relationships between the classmates as they face various challenges.