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.
Fanfiction.net also has a decent selection. You can find some Spy x Family fanfics there that deal with reveals. However, the interface might not be as user - friendly as AO3 when it comes to filtering, but with a bit of searching, you can still come across some interesting reads.
It might also be Anya herself. Since she has telepathic powers, she could accidentally let something slip while reading someone's mind. Or she could be tricked into revealing a secret by someone who realizes she has this special ability. This would create a lot of drama and conflict within the family as they try to deal with the consequences of the secret being out in the open.
Popular Spy x Family fanfics often focus on the family relationships. They expand on the interactions between Loid, Yor, and Anya. For example, they might show how they handle a family crisis or a celebration. Also, many fanfics like to explore the hidden sides of the characters. Such as Loid's real emotions behind his spy facade, or Anya's full potential with her telepathic ability.
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, in a Venti identity reveal fanfic, it could involve Venti dropping his carefree bard facade and showing his more divine or ancient side. It might include scenes where he has to use his true powers to protect Mondstadt from a great threat. And this could lead to a whole new understanding of his character by the people of Mondstadt and the readers of the fanfic.
Another possibility is that it could be a normal citizen of Mondstadt who has a special connection or intuition. This person might not be as powerful or important as Jean or Diluc, but they could have a moment of realization. Maybe they see Venti do something that defies normal human capabilities and start to suspect that there is more to him than meets the eye.
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.
There could be a fanfic where Loid accidentally discovers that Yor is extremely ticklish. This could lead to some really funny and endearing moments between them as they try to keep it a secret from Anya. In this type of fanfic, the author might play with the idea of how the two adults handle this newfound knowledge in the context of their pretend family situation.
You can try looking on fanfiction websites like Archive of Our Own (AO3). It has a large collection of fan - created works, and there's a good chance you'll find Spy x Family tickle fanfics there.