How about a crossover with a magical world? Anya discovers a magical portal in their backyard and they all end up in a world full of wizards and witches. Loid has to pretend he's just a normal human while secretly trying to find a way back home, and Yor might find that her fighting skills are quite useful in this new and dangerous world.
One idea could be to cross it with a superhero universe. For example, Anya could suddenly gain real superpowers and the Forger family has to deal with that while still maintaining their cover. Twilight might have to figure out how to use Anya's powers for his missions without blowing their family facade.
A crossover with a detective-themed series could be cool. Loid could team up with a famous detective character. They could be on a mission to solve a mystery that somehow ties into the world of Spy x Family. Anya, being the curious girl she is, might try to help in her own unique way, which could lead to some comical and exciting situations.
One idea could be that Yor and Loid have to go on a mission together where they have to pretend to be a couple in a more extreme situation than usual. For example, they are stuck on a deserted island and have to convince some dangerous smugglers that they are just honeymooners. This would force them to get closer and explore their feelings in a more intense way.
One idea could be the Forgers reacting to an alternate universe where their secret identities are all exposed at once. It would be full of chaos and interesting character interactions.
One idea could be revealing the true identities of the other agents in the spy organization that Twilight is part of. Maybe there's a hidden backstory about how they all got recruited and what their real motives are aside from the main mission.
One idea could be a fanfic where Anya discovers a new power that helps her in her spy adventures at school. Another could be a story about Yor's past as an assassin and how it affects her current family life. Maybe a fanfic where Loid has to deal with a difficult mission that involves protecting his family in a more direct way.
One idea could be a Spy x Spy fanfic where the two spies have to team up against a common enemy that is far more powerful than any they've faced before. This could lead to some interesting character development as they learn to trust each other.
For Spy x Family OC fanfiction, you could have an OC that is a famous artist in Ostania. This artist could be used by the different spy organizations unknowingly. The Forgers might need to get close to the artist to uncover a secret plot. Another idea is an OC that has a pet with some extraordinary abilities, like a dog that can detect lies. This pet could cause all sorts of comical and tense situations when it interacts with the Forgers. And then there's the idea of an OC who is a young prodigy in espionage, who challenges Loid's skills and tries to take over some of his missions.
Well, a great Spy x Family fanfiction idea might be to create a story where Loid has to go on a mission with Yor and Anya tags along. They end up in a foreign country and have to blend in while also completing the mission. This could lead to some really funny and heart - warming moments as they struggle to keep their cover and also take care of Anya.
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.
Perhaps there could be a situation where the Grimm from RWBY start to invade the world of Genlock. The Genlock team, with their diverse skills and abilities, would have to learn about the Grimm from the RWBY characters in order to fight them effectively. And in return, the RWBY characters could learn about Genlock's technology and combat strategies. For example, they could figure out how to use the Genlock's mech - like suits in their battles against the Grimm. It would be a great opportunity for character development as they interact and adapt to each other's ways.
One idea could be Deadpool trying to join the X - Men but constantly messing things up with his unorthodox methods. For example, he might start a fight with the wrong mutant during a training session because he misread the situation.