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.
Love and friendship also play a big part. Through the challenges, their relationship deepens. It could start as a friendship but as they face more and more difficulties together, it might turn into something more. For instance, in some fanfictions, they realize their feelings for each other during a particularly tough challenge that makes them rely on each other more than ever.
Another theme is teamwork. Since they are in challenges together, they need to work as a team. This often involves communication and understanding each other's strengths and weaknesses, like in many stories where izuku's quick thinking combines with melissa's technical knowledge.
One great piece of izuku x melissa challenges fanfiction is 'The Unseen Challenges'. It focuses on them facing various trials in a new hero training ground. The story does a great job of developing their relationship as they overcome each obstacle together.
Most mainstream platforms don't allow 'lemon' (adult - oriented) fanfictions due to their terms of service. But if you're looking for non - adult Izuku x Melissa Shield fanfictions, you could check out FanFiction.net as well. There are many talented writers who create great stories about their relationship, focusing on things like their friendship, adventures together, or how they support each other in the My Hero Academia universe.
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 family. Since family was a big part of the show, a lot of fanfiction focuses on expanding the family relationships.
Another theme could be friendship. Melissa Bell's friendships might be developed in fanfiction, showing how she bonds with different people, overcomes challenges together, and how these friendships shape her character.
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.
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.
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.