One great fanfic idea could be a story where Yor and Loid have to go on a mission together outside of their normal family cover. Maybe they end up in a foreign city and have to rely on each other more than ever. This would deepen their relationship and also add some exciting action and adventure elements to the story.
One characteristic is the focus on Anya's innocence. Most fanfictions highlight her child - like view of the world despite her secret powers. For example, in many stories, she might misunderstand a situation in a really cute way.
Well, a simple fanfic could just be about Yor and Loid sharing a quiet moment at home. Just them talking about their day, and slowly realizing how much they've come to rely on each other. It's a sweet and simple concept that shows the growth of their relationship.
One great Spy x Family Anya fanfiction is 'Anya's Secret Adventure'. It shows Anya having a solo adventure in the city while trying to keep her psychic powers a secret. Another is 'Anya and the School Project', which focuses on Anya's struggles and fun times with her schoolmates during a group project.
Their relationship could develop further through shared experiences. For example, if they face a big threat together to their family or mission, it would force them to rely on each other more. Another way is if they start to open up about their pasts little by little. Maybe Yor shares more about her assassin training and Loid starts to tell her about some of his most dangerous spy missions.
One idea could be a mission where Loid and Yor have to pretend to be a couple at a high - society ball for a mission, but end up having a real emotional moment. Another could be a story where Yor's assassin past comes back to haunt her and Loid has to protect her and their family identity. Or perhaps a fanfic where they go on a family vacation and deal with the challenges of keeping their true selves hidden while still having fun as a family.
A fanfiction about Anya's school adventures sounds really cool. Anya's experiences at school are already full of humor and chaos in the original work, so in fanfiction, it could be even more exaggerated. Maybe she'll get into some crazy trouble with her classmates that involves her telepathic powers in a new and hilarious way.