Saitama is a very powerful hero in the One Punch Man universe. He can defeat any opponent with just one punch. He has a simple appearance and a somewhat laid - back personality.
A more interesting plot twist could be related to the origin of these powers in the fanfiction. Maybe it's not just a random occurrence. There could be a hidden connection between Tatsumaki's world and Izuku's world. For example, some ancient prophecy or a secret experiment by an organization that spans across different anime universes. This discovery could change the whole dynamic of the story as Izuku has to not only master his powers but also figure out the mystery behind them.
First, you need to create a well - developed OC (Original Character). Think about their personality, backstory, and what makes them unique. For example, if your OC is a hero, consider their powers and how they might interact with Tatsumaki's abilities. Then, plan the plot. It could be a love story, a friendship - turned - rivalry, or a team - up adventure. Make sure to keep Tatsumaki's character traits in mind, like her pride and powerful psychic abilities.
The theme of the OC being an outsider is also quite common. Tatsumaki is part of a certain world in the original work. So when an OC enters that world, there are always elements of the OC trying to adapt, facing prejudice or suspicion, and ultimately finding their place, often with Tatsumaki's help or against her initial opposition.
With All Might, Izuku's relationship could shift. All Might would have to guide Izuku in a different way since Tatsumaki powers are not like One For All. He would need to learn new ways to train Izuku and also make sure Izuku doesn't lose his original heroic spirit while using these new powers.
You can start by looking on fanfiction platforms like Archive of Our Own (AO3). There are a large number of fanfic writers who post their works there. Another option is FanFiction.net. These platforms have a search function that allows you to specifically look for 'Oc x Tatsumaki' fanfics. Just type in the relevant keywords in the search bar, and you'll get a list of available stories.
Well, there's a fanfic called 'Saitama and Tatsumaki: A Different Dynamic'. In this story, it shows how their different personalities clash and then gradually find harmony. It's really interesting to see how the author weaves their interactions. Also, 'The Power of Two' is a fanfic that delves into how their combined powers could potentially save the world in a unique way.
A fanfiction plot could involve Tatsumaki and Saitama getting trapped in an alternate dimension. They have to rely on each other to find a way out. Along the way, they learn more about each other's pasts and what made them become heroes. This would be a great opportunity for character development and exploring their relationship in a new setting.
One idea could be that Saitama and Tatsumaki are forced to team up for a special mission. During this mission, Tatsumaki starts to see Saitama's hidden strength and unique charm, which gradually changes her initial dismissive attitude towards him.
One good tatsumaki x oc fanfiction is 'Tatsumaki's New Companion'. It has an interesting plot where the OC has unique powers that complement Tatsumaki's. The interactions between them are well - written, showing a gradual development of their relationship.