Green Goblin is another option. He might take advantage of the divided superhero community. He could target Spider - Man specifically, knowing that with the other heroes at odds, there's less chance of them coming to his aid. He could also try to use the conflict to his advantage in his criminal activities, like stealing from places that are unguarded because the heroes are busy fighting each other.
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 a Miraculous holder who has been corrupted by the power. They might start to think that they should be the sole leader and use the civil war as a means to take over. For example, if a holder of a powerful Miraculous like the Bee Miraculous turns bad, they could be a formidable antagonist.
The Green Goblin could be a great main antagonist. He has a long - standing feud with Spider - Man. Maybe in this fanfiction, he could be out for a more brutal revenge after a particularly humiliating defeat. He could target Spider - Man's loved ones or try to completely destroy his reputation in the city.
The Green Goblin could be a great main antagonist even in a Spider Man reborn story. Maybe in this version, he has developed a new form of insanity - inducing gas that affects Spider Man's new powers. Another possibility is Doctor Octopus. He could have found a way to merge his mechanical arms with some sort of alien technology, making him far more powerful and a real threat to Spider Man reborn.
It could be a corrupted version of an old ally. For example, a former S.H.I.E.L.D. agent who was exposed to some strange space radiation. This has turned him evil and he now wants to use his knowledge of Earth and his new - found space powers to create chaos. He blames Spider Man for not being able to save him from the radiation and is determined to make Spider Man pay, all while causing destruction across the galaxy.
Perhaps Doctor Octopus. But instead of just having mechanical arms, his mutation gives him the ability to control any metal object in the city. He wants to take over the city using his newfound powers and sees Spider - Man as the only obstacle.
In a fanfiction, Spider - Man could also die due to friendly fire. With the heroes split in the Civil War and the chaos of battle, it's possible that a misaimed attack from one of the heroes accidentally hits Spider - Man and causes his death.
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.