A scientist who has been experimenting on the sybiote might be the antagonist. This scientist doesn't want Peter to have the sybiote back because they believe it's too dangerous in Peter's hands. They might have their own plans for the sybiote, like using it to create an army of symbiote - controlled minions.
It could be another symbiote - powered character. Maybe someone who has also bonded with a sybiote and wants to keep Peter from getting his back because they see him as a threat to their new - found power.
In the fanfiction, perhaps a new villain has emerged who has been using the sybiote in a harmful way. Peter then realizes he needs to get the sybiote back to stop this villain. He embarks on a quest to retrieve it, facing various challenges along the way, like battling the villain's minions and overcoming obstacles in the villain's lair.
The main antagonist could be a scientist who wants to study the chibi. This scientist might be a bit of a madman, seeing the chibi as a unique specimen to experiment on and not caring about its well - being. He could try to capture the chibi and Peter has to stop him.
It could also be that some of Thanos' followers or those who try to take advantage of the post - snap chaos become the main antagonists. For example, some of the rogue elements in the universe might see this as an opportunity to gain power and territory, and they would oppose the Avengers who are trying to restore order. So they could step up as the main threats in the fanfiction.
Rocket could be another helper. With his technical expertise, he could build a device or figure out a scientific solution to bring back Peter's powers. He's always been good at tinkering with things.
It could be a new character, like a powerful wizard who has a grudge against Percy for some reason. Maybe Percy accidentally disrupted one of his spells, and the wizard is out to get him, which makes Percy mad.
It could be a lone wolf type of character. This person is a survivor in the same world as Clementine but has gone a bit crazy from all the hardships. They see Clementine as a competitor for resources and decides to shoot her to get rid of the competition.
It might be an accident - causing character. This could be someone who is clumsy or irresponsible. Their actions, even if not intentional, end up causing harm to Judy. For example, they might accidentally start a fire in a place where Judy is, and she gets burned in the process.
It could be Draco Malfoy. In some fanfictions, Draco might be pushed to such an extreme by his family's beliefs that he resorts to physical violence against Hermione. He might punch her out of jealousy or to assert his so - called superiority.
Team Rocket is an obvious choice. They could use some new, devious hypnosis device to control Ash for their own gain, like trying to steal all his Pokémon.
Dr. Eggman could be the antagonist. He's always trying to defeat Sonic, and shooting him could be part of one of his evil plans. Maybe he developed a new weapon specifically designed to take Sonic down.