Another option could be Minnie. She loves Mickey just the way he is. When Mickey starts to become a Pete fan and change his ways, Minnie could be against it. She might see Pete's more rough - and - tumble style as a threat to Mickey's kind - hearted nature. So, she tries to keep Mickey away from Pete, acting as the antagonist in this fanfiction.
Well, maybe Mickey starts to admire Pete's bravado. He could see Pete's confidence in getting what he wants and be intrigued by it. So Mickey begins to follow Pete around, trying to learn his ways.
It could be the previous Dark One who doesn't want to give up their power completely. Even though Emma has become the new Dark One, the old one might try to manipulate her from the shadows, trying to get her to make mistakes that would allow them to regain their power.
Perhaps a former teacher who always had it in for Pete. At the reunion, this teacher could be there as a guest and start criticizing Pete's life choices or bring up old mistakes he made in school. The teacher might be jealous of Pete's new - found confidence or success and wants to put him down.
It could be another ghost who is jealous of Casper's new human form. This ghost might try to sabotage Casper's attempts to fit into the human world, perhaps by causing chaos around him and making humans afraid of him again.
It could be a character who wants to use Percy - turned - Aku for their own evil purposes. Maybe a dark wizard who thinks that Aku - Percy can be the ultimate weapon to take over the world. This wizard would try to manipulate Percy's new form and powers to do his bidding.
Perhaps a powerful vampire lord who sees Sookie as a threat to his power. Since she was once human and has connections to both worlds, he might fear that she could rally other vampires and humans against him. He could try to eliminate her by any means necessary, leading to a lot of action - packed scenes in the fanfiction.
It could be Sasuke. If Sasuke has turned completely to the dark side and Naruto fails to save him, Naruto might be so frustrated and angry that it starts to make him cold - hearted towards everyone. Sasuke's actions could be the trigger for Naruto's transformation.
The Ministry of Magic could be the main antagonist. They might view Harry's fascination with Dementors as a sign of him turning to the dark side. So they try to stop him from his research and exploration of Dementors, fearing he'll use their powers for evil. After all, Dementors are associated with dark magic in the eyes of the Ministry.
It could be an old and powerful vampire who sees Hermione as a threat to his rule. Since Hermione is a very intelligent witch - now vampire - she might be able to figure out a way to undermine his power.
It could be an old and powerful vampire lord who sees Natsu as a threat to his rule. Since Natsu is a new and different kind of vampire, the lord might want to eliminate him.