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.
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.
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.