One interesting storyline could be their ideological clashes. Captain America and Iron Man have different views on things like government control and superhero registration, which leads to intense conflicts within the story.
In the comics, Iron Man did not kill Captain America. Their conflicts and battles were part of the storylines, but not to the extent of one killing the other. The relationship between them was complex but not fatal.
In the comics, Iron Man doesn't end up killing Captain America. Their battles and disputes have various outcomes, but not one that involves a fatal blow from Iron Man.
In the world of fanfiction, the idea of Iron Man killing Captain America can be quite controversial. Ethically, it goes against the typical heroic and friendly relationship they have in the mainstream Marvel universe. However, fanfiction is a space for exploring different scenarios, so while it may seem unethical to some, it can be a creative exploration for others.
A third common theme is sacrifice. Fanfictions may depict situations where either Captain America or Iron Man has to sacrifice something for the greater good or for the sake of their relationship. It could be something as simple as sacrificing personal time to train together or as big as sacrificing a chance at personal glory to save the other.
There is a fanfic named 'Cap and Iron Man: United Front'. In this, they team up against a new and powerful enemy that threatens the whole world. It's full of action scenes and also some heart - to - heart conversations between them that are really well - written.
One interesting fanfiction could be where Captain America and Iron Man have to team up in an alternate universe to fight a new threat that combines their worst enemies' powers. It could explore their relationship in a different setting, highlighting their differences and how they overcome them to save the day.