Another possibility is that the main antagonist could be the wizarding society as a whole. If Harry has been constantly marginalized or misunderstood by the general wizarding population, he could snap and see them as the enemy. Their narrow - minded views and prejudices, like against half - bloods or Muggle - borns, could be what pushes Harry over the edge. He may then try to change or even rebel against the entire wizarding society.
It could be Neji. Even though they are family, Neji has been quite tough on Hinata in the past. His overbearing nature and the way he has sometimes looked down on Hinata could be the final straw that makes her snap.
It could be someone who wants to see Harry fail in the Triwizard Tournament. Maybe a Slytherin student who is jealous of Harry's fame. They might have found a way to manipulate the Goblet of Fire to put Harry in a difficult situation, which in turn makes Harry snap.
Harry Potter and Severus Snape are the central figures. However, the other Hogwarts professors could also be important. For instance, McGonagall. She might be the one who tries to calm Harry down after he snaps at Snape or she could be the one who encourages Snape to be more understanding towards Harry. Then there are the Death Eaters. If the fanfiction is set in a context where Voldemort is still a threat, the actions of the Death Eaters could impact the relationship between Harry and Snape. They could be the catalysts that force Harry and Snape to work together and ultimately strengthen their father - son - like bond.
Well, it might be that Harry snaps due to his ongoing struggle with Voldemort. After constantly being in a state of fear and having to fight for his life and the lives of his friends, he reaches his limit. He could turn against some of his friends who he feels haven't been fully supportive or understanding. His snapping could lead to a power shift in the wizarding world, as he may start to use his magic in more aggressive and unorthodox ways to protect himself and what he believes in.
Voldemort would still be a major character. After all, he is Harry's main antagonist. If Harry snaps, it would likely be in relation to his ongoing battle with Voldemort. And the Death Eaters, as Voldemort's followers, would also be involved. They would be on the receiving end of Harry's new, more aggressive stance, and their reactions would shape the story further.
Well, it could be about Harry finally losing his patience with all the constant threats and dark magic. Maybe he decides to take a more aggressive approach against Voldemort and his Death Eaters, instead of always waiting for them to make the first move.
One possible plot could be that Harry discovers some new information about his parents that makes him see Snape in a different light. However, instead of handling it calmly, he snaps because of all the pent - up anger and confusion. For example, if he finds out that Snape had a more complex role in his parents' lives than he thought. His snapping could involve shouting at Snape in front of the whole school, which then sets off a chain reaction of events that slowly changes their relationship from one of enmity to something more like a father - son dynamic, full of misunderstandings and reconciliations.
A power - hungry merman who wants to rule over all the mer - people. He sees Harry as a threat because Harry has some special powers or a claim to the throne that he doesn't want to share.
A new dark wizard who has emerged from the shadows, someone who idolized Voldemort and is trying to pick up where he left off. This antagonist could be using new and more insidious magic that Harry has to figure out how to combat as an Auror.
A power - hungry noble could be the main antagonist. This person might be jealous of the Queen's position and wants to overthrow her, and sees Harry as an obstacle in his way.