It could also be that Tony is in a difficult financial situation and can't afford to hire Natasha at the moment. He might be cutting costs and has to choose someone with a lower salary requirement. Or, there could be some external pressure, like from a business partner or a government agency, that prevents him from hiring Natasha.
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.
Perhaps Tony has heard some rumors about Natasha that make him hesitant. Even if these rumors are unfounded, they could still influence his decision. Another reason might be that he already has someone in mind for the position who he thinks is a better fit. It could be that he has a personal acquaintance who he wants to give the job to instead of Natasha.
It could be that Tony Stark already has a full team with specific skill sets that he believes don't need Natasha's skills at the moment. He might be focused on tech - based heroes or those with superpowers that are more in line with his current projects.
Natasha could also be disappointed but determined. She might see this as a setback but use it as motivation to prove herself elsewhere. For example, she could go and work for one of Tony's competitors and become very successful there, just to show Tony what he missed out on. She could also try to improve her skills or image to make herself more appealing in the future.
Perhaps it causes a rift in the superhero community. Some might side with Tony's decision, while others might see it as a mistake. This could create tension and different factions within the community, leading to various sub - plots and conflicts.
One possible plot could be that Tony and Natasha reunite in heaven. They might look back on their time on Earth, the battles they fought together. They could share their regrets, like Natasha might regret not being able to do more for the world before her sacrifice, and Tony might regret some of the decisions he made during the various conflicts. They then start to explore heaven together, meeting old friends who also passed away and reminiscing about their shared memories.
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.
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.
A possible plotline could be them having to keep their marriage a secret from the world. Maybe because of Tony's business rivals or Natasha's undercover work. They would have to navigate their relationship while hiding it from everyone, which could lead to some really tense and exciting moments.
Maybe Tony is a very independent person. He likes to do things on his own and get things by himself. Being handed things might make him feel like he's being treated as incapable or like a child.
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 an old enemy from their time on Earth who somehow managed to get into heaven. This enemy could hold a grudge against both of them and wants to take revenge. They might use their knowledge of Tony and Natasha's past weaknesses to try and defeat them in heaven, where the rules are different from Earth.