One common theme is the development of their relationship. When they are undercover, they often have to act like they are closer than they might be in their normal work situation. This can lead to real feelings emerging. For example, in many fics, they start as colleagues but during the undercover operation, they start to see each other in a new light.
Another good one is 'The Undercover Mission'. This fic takes them undercover in a dangerous underworld setting. They have to rely on each other completely, and the story shows their trust and loyalty growing throughout the mission. It has some great character development for both Tony and Ziva.
One plot idea could be that Ziva and Tony are undercover as a married couple at a high - society event. They have to blend in and gather intel on a criminal who is using the event to launder money. Another could be that they are undercover in a dangerous cult to rescue a kidnapped agent, and during their time there, they start to realize new things about each other. A third idea is that they go undercover as rival spies in an international arms deal, but their fake rivalry starts to turn into something more real and they have to deal with their feelings while still completing the mission.
One character that would be interesting to include is Ziva's father, Eli. His complex relationship with Ziva could add depth to the story. Maybe he has some ulterior motives for their undercover mission. Also, bringing in some of the international contacts that Ziva has from her Mossad days could be great. They could provide unique help or challenges during the undercover work. And don't forget about Palmer. He could be the one who discovers some crucial medical - related evidence that helps Ziva and Tony in their mission.
Well, perhaps they are undercover in a foreign country. They have to blend in with the local culture, which leads to some humorous situations like Tony trying to learn the local dance or Ziva struggling with the language but still managing to intimidate people. And then there's the danger aspect, where they discover a terrorist cell operating within the city they are in and have to stop them without blowing their cover.
In some NCIS fanfiction, Ziva could be pregnant. Fanfiction often explores different scenarios for beloved characters. Writers might create a story where Ziva's pregnancy brings new challenges and developments to her relationship with Tony. It could be a plot device to explore deeper emotions and create more complex storylines within the NCIS universe.
Another way their relationship can develop is through secrets. Maybe while undercover, they each have a secret that they are hesitant to share with the other. But as the story progresses and they face more challenges, they are forced to disclose these secrets. This new - found trust can strengthen their relationship. For example, Tony might have a past connection to the case they are investigating that he's been hiding, and when he finally tells Ziva, she understands his motives better and they become closer as a result.
It could develop slowly. They might start as friends again, carefully testing the waters. Tony could be hesitant because of past hurts, and Ziva might be guarded as well. But as they spend more time together, they gradually become closer.
One possibility could be that it would completely change their dynamic at work. They might become more protective of each other and start to think about their future outside of NCIS. For example, Tony might become more serious and start thinking about settling down in a more permanent way. Ziva, on the other hand, might struggle with how being pregnant would affect her role as an agent.
If Ziva is pregnant in fanfiction, their relationship could become more intimate. They would have to rely on each other more. Tony might become more protective of Ziva.