One common theme is leadership. Carl shows potential for leadership from a young age, and Maggie has experience leading. Fanfictions might explore how they work together to lead the group, make decisions about where to go and what to do. Also, the theme of hope often comes up. Despite the dire situation, they keep looking for a better future and inspire others in the group to do the same.
One great fanfiction could be where Carl and Maggie go on a supply run together and face a new type of walker. They have to rely on each other's skills, Carl's shooting and Maggie's stealth. Another might be a story where they team up to protect the group from a hostile human group. Carl uses his quick thinking and Maggie her strength to lead the defense.
Unfortunately, Carl did die in The Walking Dead comics. It was a sad moment for fans. His death had a big impact on the storyline and the other characters.