Your fanfiction could start with the OC accidentally stumbling upon Todoroki in a moment of vulnerability. Maybe he's having a flashback about his father. The OC offers comfort, and this starts their relationship. Then, they could be chosen to represent their class in a hero tournament. Working side by side, they face tough opponents and their bond grows stronger. You could also add a sub - plot where other students are jealous of their newfound closeness, creating some interesting drama.
Another concept is that Todoroki was turned into a vampire by a villain's quirk gone wrong. He then has to find a way to reverse it. He teams up with Midoriya, who is determined to help his friend. They search for a mysterious old hero who might know a cure, all while Todoroki battles his inner vampire nature.
A good plot for My Hero Academia Vigilante OC fanfiction could start with the OC finding a strange quirk - enhancing drug on the streets. This leads them to investigate a new criminal organization that is creating and distributing these drugs. As they dig deeper, they realize that this organization has ties to some powerful people in the hero world. Along the way, they might meet an old friend who has turned to the dark side and is involved with the organization. They have to decide whether to save their friend or focus on taking down the organization. Another plot could be that the OC's secret identity is at risk of being exposed when a nosy journalist starts following their every move.
A good plot could involve the assassin OC having a past connection with a major villain in the My Hero Academia world. They are forced to confront this villain when the villain discovers their existence and tries to either recruit or eliminate them. Or perhaps the OC is on a mission to steal a powerful quirk - enhancing drug, but the heroes get in their way and they have to decide whether to fight or cooperate.
You could write their first meeting in a really intense situation. Like during a villain attack at the school. The OC could be trying to save some students and Todoroki comes in to help. Their eyes meet, and there's an instant connection. Or perhaps it's a more ordinary setting like in the cafeteria. The OC accidentally spills their drink on Todoroki and they start off on a bit of a rocky note, but then Todoroki is intrigued by the OC's apology and sincerity.
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.
A great plot could involve the ruthless OC being a double agent. They pretend to be a hero but are actually working for a secret villain group. As they interact with the heroes, they start to have doubts about their loyalty to the villains. Maybe they fall in love with a hero, which further complicates things. Another plot is that the OC has a vendetta against a particular hero or villain family. They use their ruthless tactics to systematically take down members of that family one by one. Also, the OC could be part of an experiment that gives them super - ruthlessness, and they have to deal with the consequences of this new - found trait.
One idea could be an OC with a unique quirk that doesn't seem powerful at first but has a hidden potential. For example, an OC who can manipulate shadows in a very limited way, but discovers they can use it to travel between dimensions in the shadows. Another could be an OC who is a foreign exchange student with a quirk that is considered abnormal in the My Hero Academia world, like a quirk that allows them to communicate with plants and use them as allies in battle. Or an OC who has a quirk that lets them see the future but only in their dreams, and they have to figure out how to use this information to help their classmates and heroes.
One idea could be a study session. Momo is known for her intelligence, and your OC could be struggling in a particular subject. They meet in the library, and Momo helps your OC study. It could be a cute and character - building scene. Another scene could be a battle simulation in school. They're paired together against other students, and they have to use their quirks in tandem. Your OC might have a quirk that enhances Momo's creation quirk in some way. Or a scene at a hero convention where they both attend to learn more about being heroes and get inspired by the pros there.
Another great fanfic is 'Shared Secrets'. In this, the OC discovers a secret about Todoroki that brings them closer together, and the author does a great job of writing their emotional connection.
There's also 'Growth Together'. This fanfic shows how Deku and Todoroki support each other during tough times at UA. They face challenges not only from villains but also from within the school. Their relationship develops as they help each other overcome these obstacles.