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.
By showing their shared experiences. For example, when they are both trying to protect Anya from something or someone. This common goal can bring them closer as they rely on each other's skills. They start to trust each other more and more with each passing situation.
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.
The potential for different storylines. The Spy x Family setting gives a lot of room for creativity. Fanfic writers can create all sorts of scenarios for Loid and Yor, from dangerous spy - assassin missions that test their relationship to simple, domestic moments that show their growing bond. There are also the added elements of Anya and the overall comical yet tense world they live in, which can be incorporated in unique ways in fanfics.
One great fanfiction could be where Loid and Yor's first kiss is a result of a near - miss mission. They think they might lose each other and in a moment of intense emotion, they share a kiss. It's a sweet and unexpected moment that deepens their connection.