A music producer could be the main antagonist. If Beca Mitchell refuses to follow the producer's unethical demands, like changing her style to something she doesn't believe in, the producer might see her as a liability. So, in order to protect their own reputation and power in the industry, they decide that Beca Mitchell must die, at least in the context of the fanfic.
One possible plot could be that Beca Mitchell has wronged someone in the story world. Maybe she betrayed a friend's trust in a music competition, and that friend then plots her downfall which could lead to her 'death' in the fanfic, whether it's a social death or something more extreme.
You can find Beca Mitchell fanfic on Archive of Our Own (AO3). It has a large collection of fan - created stories related to various characters, including Beca. Another place is FanFiction.net. There are many writers who post their Beca Mitchell fanfics there. Just search for 'Beca Mitchell' on the search bar of these websites and you'll be presented with a list of available fanfics.
One common theme is friendship. Since they start as friends in the Pitch Perfect series, many fanfics explore the deepening of their friendship. For example, how they support each other through tough times like auditions or dealing with difficult people in their lives.
Another possibility is that the wizarding legal system or a corrupt official within it could be the main antagonist. They might be trying to make things difficult for Harry, or perhaps they are in cahoots with the Weasleys and are trying to cover up the Weasleys' misdeeds. This would add an interesting layer of complexity to the fanfic.
It could be the person who spreads false rumors about Jiraiya to Naruto. This person might be an enemy of Jiraiya who wants to drive a wedge between him and Naruto. By making Naruto ignore Jiraiya, they hope to weaken both of them.
One popular Beca Mitchell fanfic is 'Pitch Perfect Dreams'. It explores Beca's journey after the events of the movies in a really interesting way. It delves into her music career and her relationships with the other Bellas. Another great one could be 'Beca's New Harmony' which focuses on her creating new music and dealing with the challenges that come with it. There are also many on fanfic platforms like Archive of Our Own that you can explore based on different themes and storylines related to Beca Mitchell.
Another good fanfic is 'The Beale - Mitchell Bond'. This one focuses more on their friendship at first and then gradually transitions into something more. It's well - written with great character development for both Beca and Chloe.
Perhaps some of the other ninja villages see Naruto as a potential danger in the future, so they manage to infiltrate Konoha and make it seem like Naruto has committed a great crime, resulting in him being left in the forest. In this case, the main antagonist would be the ninja from the other villages.
Perhaps an unknown and powerful dark ninja. This ninja could be from a long - forgotten clan with dark powers. They might have a vendetta against the Leaf Village and see Naruto as the key to their revenge. So they use their dark arts to make Naruto lose his sanity and turn him against his own village.
It could be Voldemort or his followers. They might see Dumbledore as a major obstacle in their path to power. So they could manipulate Harry, perhaps through some mind - control magic or by spreading false rumors, to get Dumbledore fired. By removing Dumbledore, they hope to gain more control over Hogwarts and the wizarding world in general.