A new vampire who is jealous of Edward's relationship with Bella could be the antagonist. With Edward having amnesia, this new vampire could try to take advantage of the situation and win Bella over. They could also try to turn Edward into a more 'evil' version of himself while he is vulnerable.
Perhaps a group of rogue vampires who have heard about Edward's amnesia and think they can use him for their own gain. Maybe they want to use his knowledge of the Cullen family or his unique vampire abilities (even if he doesn't remember them at first) for something sinister, like starting a war between vampires and humans.
A powerful and corrupt politician could be the main antagonist. He might be using his influence to cover up illegal activities, and Edward, as an honest cop, stands in his way. The politician could try to frame Edward or have him removed from the case.
It could be a character who is jealous of the relationship between Roy and Edward. Maybe someone who wanted to adopt Edward themselves or has a grudge against Roy for taking Edward in.
It could be one of the traditional Volturi members who oppose Edward's presence. They might see him as an outsider who could disrupt their long - established order. For example, Caius might be against Edward joining as he's often portrayed as more rigid and less accepting of new vampires in the Volturi.
Maybe a witch who has a vendetta against vampires in general. She could use her magic to cause all sorts of trouble for Edward, like casting spells that prevent him from using his vampire abilities or even creating illusions to drive him mad. Since she's a witch, she has different powers compared to a normal vampire hunter, so she'd be a very formidable antagonist. She might also be able to manipulate other humans or vampires to do her bidding in her quest to get rid of Edward.
One idea could be that Edward wakes up with amnesia and forgets his vampire nature. He starts living as a normal human, going to school and making new friends. This would lead to a lot of comical and also heart - wrenching moments as he slowly discovers his true self again.
It could also be a magical being from another dimension. This being might have caused Chris's amnesia on purpose in order to study the Charmed Ones' reactions. They could be interested in the power dynamics within the family and how they deal with a member who has lost their memories. Their actions could range from simply observing to actively trying to disrupt the family's harmony.
There could be an organization that wants to keep Ki a secret from the world of alchemy. They see Edward's attempts to learn Ki and potentially bridge the gap between the two as a danger. So, the leaders of this organization send their agents to stop Edward, and the most powerful of these agents could be the main antagonist in the fanfiction.
It could be another alchemist who wants to use this new energy for evil purposes. This alchemist might see Edward as a threat to his plans and constantly try to stop him from learning more about the energy.
His parents or adoptive parents in the Cullen family are likely to be main characters as well. They would be responsible for taking care of baby Edward, so they play an important role in the fanfiction.
Often, antagonists are also important characters in jasper hale and edward cullen fanfiction. These could be rival vampire clans or individual vampires with malicious intents. They create conflict for Jasper and Edward, forcing them to use their powers and work together. The presence of these antagonists can drive the plot forward and also show different aspects of Jasper and Edward's personalities as they face these threats.