The Ministry of Magic could be the main antagonist. If Harry turns evil, the Ministry would view him as a great threat. They would use all their resources to stop him. Harry, on the other hand, might see the Ministry as corrupt and in need of being overthrown. This conflict between Harry and the Ministry would make the Ministry the main antagonist in the fanfiction.
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.
It could be Umbridge. If she discovers Harry's transformation into a veela, she might see it as something unnatural and try to use it against him. She could try to expel him from Hogwarts or make his life even more miserable than she already did in the original story.
Maybe the Ministry of Magic could be the main antagonist. If Harry turns into a hybrid, the Ministry might see him as a dangerous abomination. They could try to capture him and keep him locked away, thinking that his new form is a threat to the wizarding world's order. They might not understand that Harry is still the same person inside and that his new form could actually be an asset in the fight against evil.
The obvious choice could be the creator of the evil doll. Maybe it's a rogue Death Eater who has found a new and unique way to cause trouble at Hogwarts. They use the doll as their proxy to spread fear and chaos without being directly detected.
It could still be Voldemort. Even if Harry is turning to the darkside, Voldemort might see him as a threat because Harry has his own unique power. Voldemort might not want to share his control over the dark forces with Harry, so he would become the main antagonist in a fight for dominance in the dark world.
Umbridge could also be a main antagonist. She is known for her strict and often cruel rules. If she discovers Harry in his female animal form, she would likely try to use it as an excuse to further oppress him. She might see it as a sign of abnormal magic and use it to justify her actions against Harry and those she doesn't like in Hogwarts.
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.
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.
It could be a powerful vampire clan that doesn't want Harry, a new vampire, to disrupt their ways. They might try to either kill him or force him to join their ranks against his will. They could see him as an outsider who doesn't belong in the vampire world and a potential danger to their secrecy.
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.