Well, it's not super common but it does exist. There are different reasons for this kind of plot. Maybe the writer wants to explore the consequences of a breakdown in their relationship. They had their differences in the movies, like in 'Civil War', and some fanfic writers take it to an extreme by having Tony kill Steve to show how far things can go wrong when they can't resolve their issues.
One great Steve Tony Avengers Assemble fanfiction is 'The Road to Redemption'. It focuses on Steve and Tony's complex relationship as they lead the Avengers. The story has amazing character development and some really exciting action scenes.
The theme of sacrifice is also quite common. They are often put in situations where they have to sacrifice something for the greater good or for each other. For example, in some fanfics, Steve might sacrifice his own safety to protect Tony or vice versa. And a lot of fanfics also explore the theme of trust. They start with a lack of trust due to their different personalities and views, but as the story progresses, they learn to trust each other more.
Steve might compare Tony to Howard based on their personalities or leadership styles. Maybe he points out similarities or contrasts in their decision-making.
One aspect of this fanfiction could be the exploration of the complex relationship between Tony and Steve through the lens of anxiety. Maybe it delves into how their individual anxieties affect their interactions, like Tony's fear of not being good enough conflicting with Steve's anxiety about leading the team well.
One aspect to consider is the emotional turmoil. Steve, being the more traditional and morally upright character in a sense, might have different expectations in a relationship compared to Tony, who is more of a maverick. Their divorce in the fanfiction could be a result of not being able to meet each other's emotional needs.
I really like 'A Different Kind of Team - building'. In this fanfic, Steve and Tony are forced to work closely together on a special project. It delves deep into their personalities and how they gradually understand and fall for each other. It's full of interesting dialogues and character development.
One great Steve x Tony fanfiction is 'The Avengers' Domestic Avengers' series. It focuses on their relationship in a more slice - of - life setting within the Avengers compound.
One reason could be to explore the extreme consequences of their ideological differences. In the official story, they had disputes over things like the Sokovia Accords. In fanfiction, this could be pushed to the limit, leading to Tony taking Steve's life.