One likely character could be Draco Malfoy. He's always been antagonistic towards Harry, and in the third year, he might find a new and more devious way to cause trouble for Harry. He could pretend to befriend Harry to get closer and then betray him by spreading false rumors or stealing something important from Harry.
It could be that a new character who seemed friendly at first betrays Harry during his third year. For example, a new student who was actually a spy for the Death Eaters. This student might have gained Harry's trust by helping him with his schoolwork or by pretending to be on his side during Quidditch matches. Then, when the time was right, the student could expose Harry's hiding place of important magical items or his plans to protect Hogwarts.
Revenge is also a theme that pops up. After being betrayed, some characters in these fanfics might seek revenge. It adds an extra layer of drama and conflict. Additionally, the theme of redemption can be seen. Sometimes the betrayer realizes their mistake and tries to make amends, which creates a complex and interesting storyline.
One consequence could be that the wizarding world is in shock. Dementors are usually just sent away or repelled, not killed. So, the Ministry of Magic might be very concerned about this new development and start to investigate Harry.
It could be a rogue wizard who was once a student at Hogwarts and has a vendetta against the school. He was expelled for practicing dark magic and now he's back to cause trouble. Maybe he wants to steal some powerful magical artifacts from the school's vaults.
He might be extremely angry and confused. After all, parents are supposed to be the ones you can trust the most. He could start to question everything he knew about his family and himself.
Well, it could be that in this fanfic, Harry has some sort of special artifact or weapon. For example, he might find an ancient magical object that has the power to vanquish dementors. He figures out how to use it and thus is able to kill the dementors. It could also be that he has made a special connection with some magical being or force that gives him the strength to take on the dementors in a more lethal way.
Well, a great plot for a Harry Potter third year fanfic could be centered around time - travel. Since Hermione has the Time - Turner in the third year, what if there was a glitch in it that sent some students back in time to the Marauders' era? They could interact with James, Sirius, Remus, and Peter, and this could change the course of events in the present. Also, a story where Harry starts to suspect that there is more to the Dementors than just being guards of Azkaban. He could start his own investigation, which would lead him to some dark secrets about the wizarding world's prison system.
Harry could be betrayed by a teacher in the fanfic. The teacher might believe in some false evidence against Harry. For instance, if there are some dark magic items found near Harry and the teacher, without proper investigation, blames Harry for being involved in dark magic, which is a betrayal of trust students usually have in their teachers.
One possible betrayal in Harry Potter's fourth year could be when some students believed the false rumors about Harry being a cheater in the Triwizard Tournament. In fanfiction, this could be explored further. For example, Hermione might initially doubt Harry due to the overwhelming rumors, creating a sense of betrayal for Harry. However, as the story progresses, she realizes the truth and they reconcile.