One common theme is the exploration of their relationship. Since they are both important characters in Neon Genesis Evangelion, many fanfics delve into how they interact, understand each other, and grow closer. Another theme could be their individual growth within the context of their relationship. For example, Shinji might become more confident through his connection with Rei, and Rei might learn more about human emotions.
Once upon a time, there was a Todoroki sister OC named Hana. She had the same white and red hair as her brother Shoto. Hana was often in the shadow of her powerful siblings. But she had a unique power of her own - the ability to manipulate ice and fire separately, not just together like Shoto. One day, she decided to prove herself. She entered a hero competition at school. At first, everyone underestimated her. But as the challenges went on, she showed her true strength, impressing everyone with her precise control over her powers.
One possible storyline could be that Takashi and Rei are in a post - apocalyptic world. They find a safe haven and decide to start a family. The pregnancy could bring new challenges and joys. For example, they might struggle to find proper medical care for Rei during the pregnancy. But at the same time, they could be excited about bringing new life into their difficult world.