Perhaps a new vampire character could be the antagonist. This new vampire could be jealous of Edward's relationship with some of the important Volturi members, like Aro. The new vampire might try to undermine Edward at every turn, spreading rumors or causing trouble to get him kicked out of the Volturi. They could see Edward as a threat to their own rise in power within the Volturi ranks.
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.
Another possibility is that the main antagonist could be a group outside the Volturi who see Edward's joining as a sign of the Volturi becoming too powerful. This group could be made up of vampires who have been on the fringes of vampire society and fear that the Volturi, with Edward now as part of them, will start to enforce even stricter rules. So they plot against Edward and the Volturi to try and stop their growing influence.
Maybe Edward joins the Volturi to gain more power. He might see it as a way to protect Bella in a different, more forceful way. The Volturi are very powerful, and he could learn new skills or gain access to resources that would help him keep Bella safe from other threats in the vampire world.
The Volturi Queen herself could be the main antagonist. If she views Bella as a threat to her power or the Volturi's way of life, she would be a very powerful foe. She could use her influence over the Volturi guards and her own powers to try and stop Bella.
Perhaps there is a rogue vampire within the Volturi's ranks who doesn't want Bella and Alice there and tries to undermine their purpose. This vampire could be jealous of their relationship or afraid that they will expose some of the Volturi's secrets.
Caius is another important character in such fanfictions. He is usually portrayed as quite strict and unforgiving, and his reactions to Bella joining the Volturi can range from suspicion to begrudging acceptance. Also, Jane, with her special ability, has interactions with Bella that can be both menacing and oddly fascinating.
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 senior officer who has a grudge against Percy for some reason. Maybe Percy accidentally made him look bad in front of the higher - ups early on.
There might be a supernatural or magical antagonist, even in a military setting. This could be a dark wizard or a creature that has infiltrated the military for its own malicious purposes. Harry, with his experience in the magical world, would be the key to dealing with this threat, but he has to convince the non - magical military personnel of the danger first.
Perhaps an old enemy of one of the members of Team 9 shows up. Since Naruto has joined the team, they might see it as an opportunity to get revenge. This antagonist could be someone who was once defeated by the member's family and now wants to prove themselves. They would be very cunning and use their knowledge of the team member's past to try and divide the team.
It could be Voldemort. Imagine if he somehow found a way to cross over into the world of the Fellowship. He would be a powerful and menacing antagonist, using his dark magic to oppose both Harry and the Fellowship.
It could be Voldemort. Even if it's a fanfiction where Harry joins Tom, Voldemort's future self might still be a threat and oppose Harry's attempts to change Tom.