Well, a common theme is Shinji's battle against his own insecurities. Fanfic writers like to expand on how he overcomes or succumbs to these feelings. His relationship with his father is also a big one. Writers might rewrite it in a more positive light or explore the full extent of the damage it has caused him. And then there's the theme of Shinji finding his place in the world outside of the Eva battles.
One popular theme could be their relationship development. It might explore how they support each other in the complex world of Evangelion. Another theme could be their adventures together, perhaps facing new Angels or threats in the rebuilt Evangelion universe.
Another theme is powerlessness. In many of these fanfictions, Shinji snaps because he feels he has no control over his life or the situations around him. It's like a pressure cooker situation where the lack of power finally makes him break.
One common theme is despair. In these fanfictions, Shinji often experiences a much deeper level of hopelessness compared to the original story. For instance, he may feel that there is no way out of his difficult situation, whether it's his relationship with his father or his role as an Eva pilot.
One popular theme is the exploration of their complex relationship. Given their different ages and positions in the story, fanfics might delve into how they navigate their feelings for each other. For example, some might focus on how Shinji looks up to Misato as a mother figure at first but then develops different kinds of feelings as he grows.
I'm not sure if it's popular. It depends on the size of the fandom and the interests of the fans. If there are a lot of people who like the dynamic between Misato and Shinji and also are interested in more adult - themed fanfiction, it could be somewhat popular within that niche. But it might also be a very specific and not so widely known type of fanfiction.
One popular trope is the 'protective Shinji' trope. Since Rei can seem vulnerable at times, Shinji is often written to be very protective of her. This can be seen in situations where they face danger together. Another trope is the 'Rei's discovery of emotions through Shinji'. As Rei is not fully in touch with her emotions in the original, fanfiction writers like to explore how Shinji can be the key to her unlocking a range of feelings like love, jealousy, and friendship.
The popularity of shinji x rei fanfiction lies in the complex relationship between the two characters in the original work. Their relationship is full of ambiguity and potential for development, which gives fanfiction writers a lot of room to explore.
One common theme is friendship development. They start off as acquaintances or friends and gradually become closer. Another theme is overcoming challenges together. Whether it's external threats or internal struggles, they support each other.
One common theme is redemption. Characters, especially Shinji, often seek to atone for past mistakes or find a new meaning in life within the context of the Gunslinger Girl world. Another theme is the power of relationships. The bonds between Shinji and the Gunslinger Girls can be a source of strength or conflict.
A common theme could be adventure. The characters might embark on exciting journeys in the world created by the 'shinji gun girl' concept.