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 great Loid x Yor fanfic is 'A Spy's Heart'. It delves into the deeper emotions of Loid as he starts to truly care for Yor beyond their fake marriage setup. It shows his internal struggle between his mission and his growing feelings.
The 'Protectiveness' trope is also common. Given Loid's role as a spy and Yor's secret identity as an assassin, they often find themselves in situations where they need to protect each other. Fanfics play on this, showing their concern and the lengths they'll go to keep the other safe.
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.