There's also a possibility that a new, original villain could be introduced. For example, a scientist who was wronged by the superhero community in the past and now wants revenge. He could create powerful weapons and technology that can counter the superheroes' powers, and he uses the divide in the Civil War to pick them off one by one.
The Kingpin could be a great main villain. He could see the Civil War as an opportunity to expand his criminal empire. He could be secretly funding both sides in a way that makes the conflict worse, all while Spider - Man is trying to figure out his role in the war.
A new supervillain could emerge. Maybe a scientist who got access to some of the leftover technology from the Civil War battles and uses it to create a powerful weapon. He then targets Spiderman because he represents the superhero community.
It could be Mysterio. He's known for his illusions and deception. In the context of a civil war, he could use his powers to create false scenarios that make the heroes fight each other even more. He might pretend to be a hero on one side, but secretly be working to make the conflict worse for his own gain.
A really interesting plot twist might be if Spiderman's identity is somehow revealed not by his enemies but by a so - called friend within the superhero community during the events of the fanfiction. This could lead to a whole new set of conflicts as he has to deal with the fallout.
Peter Parker, of course, is extremely popular. He's the central character in most of the fanfiction related to this. His struggle to choose sides and his moral compass are often explored in great detail.
Loki could be a great main villain. He could be secretly fueling the Civil War for his own gain, like trying to take over Asgard while everyone is distracted. He has the cunning and the motivation to do so.
Vision is a character that can be quite popular in this type of fanfiction. His unique abilities and his own thoughts on the Sokovia Accords can create interesting storylines with Spiderman. He can be a source of knowledge and power that Spiderman might look up to or have to oppose, depending on the plot of the fanfiction.
Spiderman could be portrayed as a mediator. He'd swing between the two sides, listening to their grievances and trying to find common ground. His actions would be quick and decisive, using his agility to dodge attacks and get in between feuding heroes.
It could be a corrupt government official who is using the Civil War Accords as a means to gain more power. This person might be secretly manipulating the situation to get rid of heroes who oppose their regime, and Captain Britain would have to expose their true intentions.
There's a fanfiction called 'Young Spidey in Civil War'. It's really good because it explores Spiderman as a young and inexperienced hero in the complex situation of the Civil War. It shows his growth and how he tries to navigate the difficult political and moral landscape of the war between the Avengers.