It could be a new villain that we haven't seen in the original Marvel universe. This villain might be able to manipulate Natasha's emotions and loyalty, making her betray Tony. For example, a powerful sorcerer who uses dark magic to control her.
Maybe in the fanfiction, Natasha betrays Tony because she has been brainwashed by an enemy organization. They manipulate her mind to turn against Tony and his Avengers team. This could lead to a lot of action and drama as Tony tries to figure out what's wrong with her and how to save her.
She would likely become angry. Natasha is a strong character, and being betrayed would stir up her fury. She might start planning her revenge, using her skills and intelligence to get back at Percy. Maybe she'll try to expose his true motives to others in their world or group.
One common theme is misjudgment. The team misjudges Tony's intentions, leading to their betrayal. For example, if Tony is working on a secret project to save the world but keeps it from the team, they might think he's up to no good.
One possible reason could be that Tony has some pre - existing biases. Maybe he has a set idea of the type of person he wants on his team and Natasha doesn't fit that mold in his mind. For example, if he's looking for someone with a more technical background and Natasha's skills seem more combat - oriented to him at first glance.
Another theme is teamwork. They are both part of the Avengers, so many fanfictions focus on how they work together on missions, whether it's fighting a new villain or rescuing hostages. Their different skills - Tony's technology and Natasha's espionage abilities - make for interesting team - up scenarios.
Natasha may come to view Tony as a kindred spirit. If she discovers that he also has a longing for real connection like she does, she'll feel a sense of kinship. His true nature might show that he is not as different from her as she initially thought. They could then develop a closer friendship or even something more, as they realize they share common emotional ground beneath their outward appearances.
One example could be a story where the team betrays Tony over a new technology he's developing. They might think it's too dangerous and try to stop him by underhanded means, like leaking information about his project to his rivals.
One common theme is the blending of their two very different personalities. Tony is often portrayed as more flamboyant and tech - savvy, while Natasha is more reserved and stealthy. Their marriage forces them to find common ground and understand each other better.
One common theme is the fear of the unknown. Since they are both superheroes, they're not used to dealing with normal life events like pregnancy. Another theme is the support they give each other. For example, Tony might use his resources to ensure Natasha has the best medical care during pregnancy.