Well, if Spider - Man died in a Civil War fanfiction, it could be that his death serves as a catalyst for the end of the war. His death might make both sides realize how pointless their fighting is. Maybe the heroes would then come together to honor his memory and try to make amends. His friends in the superhero community, like Ned if he knew about Peter's identity, would be heartbroken. And villains might take advantage of the situation, thinking the heroes are weakened without Spider - Man, which could lead to new threats emerging in the fanfiction world.
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 scenario could be that Spider Man sacrifices himself to save the city from a powerful and unstoppable villain. He might use all his strength and powers to defeat the enemy, but in the process, his body gives out due to the extreme exertion.
He could become more withdrawn. Losing Aunt May, who was like a mother to him, would be a huge blow. He might isolate himself from his friends and even from his crime - fighting duties for a while as he deals with his grief.
It could be Doctor Doom. He might be seeking to take over multiple universes in a crossover. Spider Man stands in his way, and Doom, with his superior intellect and powerful magic, manages to defeat Spider Man. His death would then be a huge shock to other heroes in the crossover, like the Avengers or the Fantastic Four who then have to avenge him.
In some fanfictions, Peter might die heroically while saving the city from a powerful villain like Venom. He could sacrifice himself to protect his loved ones and the innocent.
New enemies could emerge. Maybe some villains would see his vulnerability after Aunt May's death and try to take advantage. As for alliances, he might team up more with other superheroes like Iron Man. He'd need more support now that he's lost his main source of emotional support at home.
Well, one possibility is that a new hero could emerge in his place. Maybe one of his friends or a previously unknown character with powers would step up to protect the city. For example, Miles Morales could become even more prominent earlier than in the regular storyline. The city would be in chaos without Spider - Man, so the police might have to step up their efforts in a new way.
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.