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.
A new, original character could be the main villain. For example, a powerful Asgardian who has been overlooked for power and decides to start the Civil War to overthrow the current regime. This character could have unique powers and a complex backstory that ties into the events of the fanfic.
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 powerful dementor - like being that has been corrupted by Asgardian dark magic. This entity could be draining the life force of both wizards and Asgardians. It would be up to Thor and Harry Potter to find a way to stop it, as it poses a threat to both of their worlds.
Sure was. Thor made an appearance in the Civil War comics and had an impact on the storyline.
A padawan who has just come of age and is now thrust into the chaos of the civil war could be a main character. Their inexperience and fresh perspective could add an interesting dynamic.
Mr. Sinister could be a great antagonist. He's always interested in mutant genetics and power. He could be the one secretly pulling the strings, making Cable and Deadpool believe they need to fight over the future of mutantkind. He could plant false evidence or use his mind - control abilities on some of the supporting characters to fuel the conflict.
It could be a power - hungry politician who wants to use the Hulk as a weapon to gain more control during the Civil War. This person might not care about the destruction the Hulk could cause, just as long as they achieve their own goals.
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.
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.
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.
Well, it could still be Thanos. His goal of collecting the Infinity Stones would be a great threat to Superman's world as well. Superman would have to figure out how to stop Thanos from using the stones' power on a galactic scale.