One interesting plot twist could be that Snape's 'mastery' over Harry is actually a ruse to protect him from a greater evil. Snape, in his complex way, is using this guise to keep Harry close and train him in secret arts that Dumbledore didn't have the time or the nerve to teach Harry. This would turn the whole master - slave dynamic on its head and show Snape's true loyalty to Harry and the cause against Voldemort.
One possible plot twist could be that Snape has a hidden agenda all along. Maybe he's been using Harry as a pawn in a bigger scheme related to the wizarding world's power struggle. Another could be that Harry discovers a long - lost family connection to Snape that changes their dynamic completely.
One interesting plot twist could be Snape secretly protecting Harry throughout the Triwizard Tournament in a more obvious way in the fanfiction. Maybe he uses his potions skills to give Harry an edge without Harry realizing it's Snape helping him.
A great plot twist might be that the fight is a test set up by Dumbledore's spirit. He wants to see if Harry can overcome his prejudice towards Snape. So, it's not a real fight to the death, but rather a test of character. Harry realizes this mid - fight and has to make a choice about how to proceed. This would add a whole new layer to their relationship.
I remember reading 'A New Dynamic' which is a great 'Master Harry Slave Snape' fanfiction. It really plays on the idea of them being in such opposite roles. Harry's character growth as he tries to handle Snape in this new relationship is really well - written, and Snape's begrudging acceptance of his situation makes for an engaging read.
A plot idea might be that Snape, in taking Harry as an apprentice, is actually trying to atone for his past mistakes. He trains Harry in a very strict but effective way, and they end up bonding over their shared experiences with Voldemort. Harry could learn how to better control his emotions and magic from Snape, which helps him in future battles.
Perhaps in the fanfiction, Draco Malfoy turns out to be a double agent. He pretends to be on the side of the Death Eaters like his family, but in reality, he is secretly helping Harry Potter and the Order of the Phoenix. Snape discovers this and has to decide whether to expose Draco or use his secret to their advantage. This would create a lot of tension and unexpected alliances.
The role reversal is a big part. We're so used to seeing Snape as the strict teacher and Harry as the student. So when Harry is the master and Snape is the slave, it's a complete turnaround. It allows for new interpretations of their characters and their relationship.
One plot idea could be that Harry, as a pet in this fanfiction universe, has some unique magical abilities that only Snape can understand and help him develop. For example, Harry might be able to communicate with magical creatures in a special way. Another idea is that Snape takes Harry under his wing as a pet to protect him from the real threats in the wizarding world that others don't see. Maybe there's a prophecy related to Harry's 'pet' status that Snape is trying to figure out.
At first, the relationship might be filled with tension. Harry would be resentful of being in a slave - like position to Snape. But as time goes on, he realizes that Snape has a depth of knowledge and a hidden agenda that might not be as bad as he initially thought. Snape, on the other hand, might start to see Harry not just as a tool or a burden, but as someone with potential that he can shape in his own way.
One possible plot twist could be that Snape blackmails Harry into joining a secret magical society at Hogwarts that Snape is leading. This society could be dedicated to protecting a powerful but dangerous magical artifact. Harry initially resists but realizes the importance of the cause as the story progresses.