One idea could be exploring the backstories of the villains in more depth. For example, how did Shigaraki develop his nihilistic view? Another could be a 'what if' scenario, like what if the villains were given a chance at redemption early on. Maybe a story where they form an uneasy alliance with some of the heroes for a common greater threat.
One idea could be that Deku gets tired of being constantly underestimated and bullied despite his efforts to be a hero. So, he decides to use his knowledge of hero strategies against them and becomes a villain. He could form his own group of outcasts who were also treated unfairly in the hero - centered society.
One plot idea could be a villain who is secretly a double agent, working for the heroes all along. They infiltrate the villain group and gather information. Another idea is a hero who loses their quirk and has to rely on their wits and combat skills alone, like Batman in the DC universe. And a villain - hero team - up to face a greater threat that neither can handle alone.
One idea could be that the 'Villain Class 1A' has to team up with the real Class 1A to fight a greater threat. Maybe a new super - villain emerges who is too powerful for either group alone. This would lead to some really interesting character dynamics as they have to put aside their differences.
Another plot could involve Ochako being manipulated by a charismatic villain. This villain convinces her that the hero society is corrupt and that by joining the 'dark side' she can actually make a more positive change. She then uses her gravity quirk to disrupt hero operations, like making heroes' equipment float away during battles.
One plot idea could be that your OC is wrongly accused of being a villain because their methods of killing villains are too extreme for the hero society. They then have to prove their innocence while still continuing to hunt down the real villains. Another idea is that a powerful villain organization puts a bounty on your OC's head, and they have to fight off assassins while also taking on the main villains.
One idea could be that the Avengers have to form study groups to learn about new threats. Captain America might struggle with modern technology in his studies while Iron Man aces all the tech - related courses. Another is that the villains also attend the same school, undercover, and try to disrupt the superheroes' academic progress.
One idea could be to create a new character who has a unique semblance that challenges the existing power dynamics in the Huntsman Academia world. For example, a semblance that can temporarily nullify other people's powers.
One idea could be a crossover where the students from Little Witch Academia visit UA High. They could participate in a joint training exercise. The witches could use their magic in creative ways during combat scenarios, while the heroes-in-training show off their quirks. It would be a fun mix of different powers and personalities.
One idea could be that Deku, turned villain, challenges Bakugou's view of heroism. He could expose the flaws in the hero society that Bakugou blindly believes in. For example, Deku might target the way heroes are ranked and how it creates a cut - throat environment. This would force Bakugou to re - evaluate his own pursuit of being the top hero.
A more relaxing plot could be a beach vacation where the characters just unwind. They could build sandcastles, have picnics, and share stories. But of course, there could be some small challenges like dealing with a sudden rainstorm or a seagull stealing their food.