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.
A more complex plot could be that Ochako has an accident that causes her quirk to go out of control. The hero society blames her and ostracizes her. In her despair, she turns to villainy. She might form an alliance with other outcast villains and try to take revenge on the heroes who wronged her by using her now - enhanced and unpredictable quirk.
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 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 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.
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.
If Ochako becomes a villain, her relationship with Deku would be severely strained. Deku, being a hero - in - training, would be conflicted between his feelings for her and his duty to stop villains. He might try to reason with her at first, but if she continues on her villainous path, they could end up enemies.
Well, in such a fanfiction, it could be due to various reasons. Maybe Ochako realizes that their hero paths are leading them in different directions. It might also be a plot device to create more drama and character development for both Deku and Ochako.
A really cool storyline could be a time - travel one. Izuku and Ochako go back in time and meet their younger selves. They have to guide their younger selves to become better heroes without changing the future too much. Along the way, they see how their past actions and decisions led them to where they are now. And this experience also makes them realize how important they are to each other in their present lives.
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 Mustard discovers a new way to use his quirk that gives him an edge in the villain world. For instance, he finds a way to combine his gas - based quirk with some form of mind - control element. This could lead to a story where the heroes have to figure out how to counter this new threat.
One plot idea could be that the Saiyan OC is mistaken for a new type of villain at first. As they try to prove they're not a threat, they start to make friends and learn about heroism. Another idea is that they discover a connection between Saiyan powers and some of the hidden secrets in the MHA world. Maybe there's an ancient power source that only a Saiyan can unlock.