Perhaps a former teacher who always had it in for Pete. At the reunion, this teacher could be there as a guest and start criticizing Pete's life choices or bring up old mistakes he made in school. The teacher might be jealous of Pete's new - found confidence or success and wants to put him down.
A Pete Lattimer high school reunion fanfiction could be full of interesting scenarios. He may find that the popular kids from high school are not so impressive anymore. There could be a mystery element, like something that happened in high school that was never fully resolved and now resurfaces at the reunion. Pete could be the one to solve it, showing his growth and intelligence. He might also make new connections with people he never really knew in high school, expanding his social circle in this fictional reunion world.
He might meet his old best friend at the reunion. They could catch up on all the years that have passed.
It could be a new teacher who is secretly a minor villain from the Percy Jackson universe. This teacher might be trying to undermine Percy's confidence and expose his demigod side to the mortal world. They could set up difficult situations for Percy in class, like impossible tests or false accusations of cheating. Also, they might try to turn Percy's friends against him by spreading rumors. The teacher could be working for a bigger bad guy in the background, but their main goal is to make Percy's high school life miserable.
It could be an old muggle acquaintance who is jealous of Hermione's success in the wizarding world. Maybe they sense that she's different and are determined to find out her secrets and expose her.
One idea could be that Pete goes to the reunion hoping to see his old crush. When he gets there, he discovers that the crush has changed a lot but he still feels a connection. Another could be Pete showing up as a big success and making his former bullies jealous. Maybe he gives an inspiring speech about how high school doesn't define you.
Another option could be Minnie. She loves Mickey just the way he is. When Mickey starts to become a Pete fan and change his ways, Minnie could be against it. She might see Pete's more rough - and - tumble style as a threat to Mickey's kind - hearted nature. So, she tries to keep Mickey away from Pete, acting as the antagonist in this fanfiction.
Loki could be a great main antagonist. He could be the mischievous student who's always causing trouble for the Avengers. He might use his magic to play pranks on them or steal their homework to make them look bad in front of the teachers.
It could be a jealous classmate. Maybe someone who has a crush on Armin and sees Annie as a threat to their relationship. This person could try to spread rumors about Annie to make Armin stay away from her.
It could be a new demigod who has turned to the dark side. Jealous of Percy's popularity and Artemis' power, this demigod could try to create chaos in the high school and turn students against them.
The school principal could also be an interesting antagonist. Imagine if he or she has somehow been influenced by a dark power from the mythological world. They start implementing strict and unfair rules at the school that target the demigods specifically, and Percy has to figure out a way to break the influence without getting expelled.