Another possibility is a group of traitorous Narnians. They might have become disillusioned with the old ways of Narnia and decided to side with an evil force. When Peter returns, they see him as a threat to their new plans and try to oppose him at every turn.
The White Witch could be resurrected as the main antagonist. She was a powerful and evil force in the original Narnia stories, and her return would pose a great threat to Peter and Narnia once again.
Perhaps Peter returns to Narnia and finds that time has passed differently there. His old friends have aged much more than he expected, and there are new political tensions in the land. He then has to use his leadership skills from his previous time in Narnia to help resolve these issues and bring peace back to the kingdom.
The Green Goblin could be a main antagonist. He's known for causing chaos in New York City, and a building collapse would be right up his alley. He could be targeting the building for some strategic reason, like trying to get to Peter Parker or to create general mayhem.
Maybe it's a group of intergalactic criminals who are after the power that Peter Parker has as a 'child of the universe'. They could be stealing artifacts from different planets to gain more power and they see Peter as the ultimate source of power that they need to complete their collection.
The Green Goblin could be a main antagonist. He might see Peter's control of Alchemax as a threat to his own criminal empire and try to take over the company or destroy it. Another could be Doctor Octopus. With his brilliant mind, he may want to use Alchemax's resources for his own mad experiments.
Some of the more powerful and vengeful spirits in Tartarus could be the main antagonists. These spirits could possess demigods or mortals close to Percy to get to him. They might be angry that he escaped their domain and want to drag him back. Additionally, there could be a group of rogue demigods who believe that Percy has been corrupted by Tartarus and they see it as their duty to stop him, even if it means becoming his enemies.
Perhaps some powerful organizations or institutions. If Peter is trying to disrupt the status quo, these entities that benefit from the current system would be his antagonists. They might see him as a troublemaker and try to stop him. They could use their resources and influence to try and take Peter down, creating a lot of conflict in the fanfiction.
One possible antagonist could be Sabretooth. He might see Wolverine's newfound fatherly relationship with Peter as a weakness and try to use Peter to get to Wolverine. Sabretooth could be jealous of the bond Wolverine is forming and want to destroy it.
It could be a new group of rogue shinobi who have been causing chaos in the ninja world while Naruto was away. They might have targeted Naruto's friends or family, which is why he's back for revenge.
Venom could be a formidable antagonist. He might be attracted to the chaos and different powers at Monster High. He could start infecting some of the students, turning them into his minions. Additionally, a group of anti - monster humans from Peter's world could find out about Monster High and decide to invade it. They see the monsters as a threat and want to 'cleanse' the school, which would put Peter in a difficult position as he has to protect his new friends from his own kind.
The main antagonists could be the official Avengers themselves. Since Peter Parker has joined the rogue Avengers, the main Avengers group might see them as a threat to the order they are trying to maintain. They could try to stop the rogue Avengers from achieving their goals, which would put them in direct conflict with Peter and his new allies.