Romance, of course, is a huge theme. How they fall in love, overcome obstacles together, and support each other in their hero careers. It could involve things like Izuku helping Mount Lady with her insecurities about her hero image, and her in turn helping him gain more confidence in his abilities.
Another place to look is FanFiction.net. However, the quality can vary a lot. You'll need to sort through the results. Some stories might be really interesting and creative, while others might not be as good. But it's still a large platform with a lot of fan - created content.
One common theme is growth. They both face challenges that make them grow as heroes and as individuals. For example, they might have to learn new skills to overcome an obstacle.
One common theme is the transformation of their relationship from rivals to lovers. They start off being competitive and then gradually see each other in a new light. Another theme is the exploration of their individual growth as they are in a relationship. They learn from each other and become better versions of themselves.
Often, the theme of acceptance is strong. Izuku has to deal with a world that sometimes doesn't accept quirkless individuals as heroes. Jirou can play a role in making him feel accepted and valued. And vice versa, Izuku can accept Jirou for who she is, quirks and all. This mutual acceptance can lead to a deep emotional connection between them, which is often explored in fanfictions.
One common theme is friendship. The stories often explore how Trish and Lady form a deep bond and support each other through various challenges. Another theme could be self - discovery. As they interact, they might learn more about themselves. And of course, there can be a theme of loyalty, showing how they stay true to each other no matter what.
One common theme is friendship. Many fanfictions explore how Thomas and Lady, coming from different backgrounds, form a strong bond of friendship. For example, they might learn from each other's strengths and weaknesses.
One common theme is the test of Izuku's heroism in captivity. Even when kidnapped, he might try to save other captives or find a way to stop the villains. Another theme could be the exploration of his relationships. For example, how his friends and mentors react to his kidnapping and how they strive to rescue him. Also, it could be about Izuku's growth in a difficult situation. He may learn new skills or gain a deeper understanding of his powers while being held against his will.
A frequent theme is the crossover of their worlds. How would Izuku fit into the world of Academy City where A Certain Scientific Railgun is set? Writers like to play with the idea of him adapting to the different rules and technology there. Also, the theme of facing challenges together is common. They might have to battle a common enemy or overcome some sort of prejudice between their two groups of friends.
The theme of the underdog and the idol is also common. Izuku is the underdog, and the female pro hero is someone he looks up to. Their relationship then blurs the line between admiration and love. For example, if it's with Mirko, Izuku admires her brash and fearless fighting style. As they interact more, their relationship can change from that of a fan - idol dynamic to a romantic one. There can also be a theme of sacrifice, where either Izuku or the female pro hero is willing to sacrifice for the other in a dangerous situation.
A common theme is their shared passion for heroics and invention. They both have a drive to make a difference in the hero world, and fanfictions often explore how they combine their skills. Another theme is friendship. Since they are classmates, many stories show them becoming good friends and relying on each other.