It could be the Green Goblin. He might manipulate Spider Man in some way, perhaps by tricking him into believing that the only way to protect the city is to rule it with an iron fist. This would slowly corrupt Spider Man's good intentions.
Another possibility is a new, original villain created specifically for the fanfiction. This villain could be a master of psychological warfare, preying on Spider Man's insecurities and doubts, until he finally breaks and turns evil.
Doctor Octopus could also be the main antagonist. He could use his scientific knowledge to create a situation where Spider Man is constantly faced with moral dilemmas that push him towards the dark side. For example, by making him choose between saving a few people or the entire city, and each choice has some sort of evil consequence.
The main antagonist could be Doctor Octopus. He might see Peter's new form as an interesting specimen to study and wants to capture him for his own evil experiments. He could use his mechanical arms to try and trap Peter in his spider form.
Dumbledore could be the main antagonist. Harry might see Dumbledore as having used him all along for some grand plan. If Harry turns evil, he would want to break free from Dumbledore's influence. Dumbledore, with his power and wisdom, would then try to stop Harry from causing too much harm, thus becoming the main opposing force.
One possibility is that he gets betrayed by those he trusted most, like his friends or the city he protects. This betrayal could plant seeds of anger and resentment, gradually turning him evil. For example, if the public blames him for something he didn't do, he might start to think that they don't deserve his protection.
It could be Yuno. Normally they are friends, but if Asta turns evil, Yuno might be the one to oppose him. Yuno has his own strong magic and a sense of justice that would make him a natural adversary.
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.
Dr. Eggman could still be the main antagonist. Even if Tails turns evil, Eggman might see him as a threat to his own evil plans. He could try to stop Tails while also taking advantage of the chaos Tails is causing among Sonic and his friends.
It could be an old enemy of Spyro who has been plotting his downfall for a long time. This antagonist might be another dragon who is jealous of Spyro's popularity and power at the academy. He could manipulate the situation to make Spyro turn evil and then use him as a weapon against the Skylanders.