Perhaps the fanfiction is set in an alternate universe. Captain America is on a mission that goes wrong and he's targeted by a group of super - villains. Spider - Man gets wind of the situation and races to help. He ends up getting caught in a web of traps and attacks while trying to rescue Captain America. In a final act of self - sacrifice, he gives his life to ensure Captain America's safety. This would then cause a ripple effect in the story, with characters dealing with the loss and maybe even seeking revenge for Spider - Man.
There are many fanfiction platforms out there. You could start by checking websites like Archive of Our Own (AO3). Just search for 'Spider - Man dies saving Captain America' in their search bar. There's a good chance you'll find some relevant fanfictions, but it might take some time to sift through the results as there could be a large number of stories.
If it's a crossover with the Fantastic Four, perhaps a cosmic entity is attacking Earth. Spider Man teams up with them to stop it. But during the battle, he's exposed to some deadly radiation from the entity. Despite the Fantastic Four's efforts to save him, his body can't withstand it, and he dies. His death could lead to the Fantastic Four creating new safety protocols for future battles.
One possible reason could be a misunderstanding. Maybe there was some misinformation spread that made them think the other was a threat. For example, a villain could have framed Spider - Man as being a danger to the city, and Captain America, being the protector of the people, decides to take him on. Or it could be the opposite, with Spider - Man believing Captain America has gone too far in some mission - related decision.
One possible fanfiction could be about Spider - Man teaming up with Captain America. Maybe Peter Parker looks up to Cap and gets the chance to fight alongside him against a new threat in New York. For example, a group of villains who have stolen some high - tech weapons and are causing chaos. Spider - Man's agility combined with Captain America's leadership and combat skills would make for an exciting story.
One common theme is friendship. Since they are both heroes from the Marvel universe, fanfictions often explore how they become friends and support each other. Another theme is mentorship. Captain America, being from an older era, can sometimes be portrayed as a mentor to Spider - Man.
A recurring theme is the contrast between their personalities. Captain America is more of an old - fashioned, by - the - book hero, while Spider - Man is young and impulsive. Fanfictions often explore how they deal with this difference. Also, the theme of sacrifice is common. They both are willing to sacrifice themselves for the greater good, and this is often highlighted in the stories. For instance, in some fanfictions, one of them will put themselves in danger to save the other or civilians.
The plot might involve an interdimensional force that accidentally merges the worlds of Spider - Man and Captain Marvel in a way that forces them into a shared adventure. They could be in a situation where they have to protect a group of innocent people caught in this strange merge. Since they are from different hero backgrounds, they initially clash but then gradually learn to work together as they face greater threats.
Another ending could be a more tragic one. With Captain America's death, the Avengers lose their moral compass. They become disorganized and are ultimately defeated by Thanos. This would show the huge impact Cap had on the team.
A simple idea could be a friendly sparring match between Spider - Man and Captain America. They both have different fighting styles, and it would be interesting to see how they interact during a non - life - threatening combat situation. Maybe they exchange tips on how to improve their fighting techniques.
One great fanfiction could be about Spider - Man's internal struggle during the events of 'Captain America: Civil War'. He's a new addition to this complex world of superhero politics. He might be initially excited to be part of the fight, but then starts to question the motives of both sides. Maybe he tries to find a middle ground, using his unique perspective as a young hero who hasn't been jaded by years of superhero rivalries.