One interesting plot idea could be that Snape discovers a hidden weakness of Voldemort. Maybe it's a long - forgotten spell that can counter Voldemort's most powerful dark magic. Snape, using his extensive knowledge of the dark arts and his own skills as a wizard, decides to face Voldemort alone to end the reign of terror. Another plot could involve Snape having to team up with some unexpected allies, like a group of students he has been secretly training, to take on Voldemort.
Well, in a fanfiction about Snape fighting Voldemort, perhaps Snape finds a way to enter Voldemort's mind completely and disrupt his plans from within. He could use his Occlumency skills in a new and extreme way. He might start by infiltrating Voldemort's inner circle, pretending to be on his side while actually working to undermine him. As the battle unfolds, Snape uses his knowledge of Voldemort's insecurities and fears, which he has learned over the years, to gain the upper hand in the fight.
Another interesting plot could be a backstory about how Snape first got involved with Voldemort. Maybe it was out of a thirst for power initially, but then his views changed over time. This could be delved into in great detail, showing his transformation from a potential dark wizard follower to a hero in the fight against Voldemort.
One idea could be that Snape endures the torture while secretly passing on vital information to the Order of the Phoenix through a magical link. Another could be that his past with Voldemort is explored during the torture, like how he first got involved with the Dark Lord and his internal struggle between loyalty to Voldemort in the past and his newfound loyalty to Dumbledore.
Perhaps a plot where Voldemort discovers that Lucius has been having doubts about his leadership. He then tasks Snape with spying on Lucius. Meanwhile, Harry, through some magical accident, gets involved in this web of intrigue. The story could be about how Harry uses this situation to his advantage to try and bring down Voldemort's regime from within.
One plot idea could be that Snape kidnaps a student he suspects of being involved in a dark magic plot against Hogwarts. He wants to question the student privately to get to the bottom of it. Another could be that he kidnaps someone he believes has information about Voldemort's next move, thinking he can protect that person and extract the information at the same time. Or perhaps he kidnaps a character in a fit of jealousy or to protect them from a greater danger he foresees.
Well, a possible plot could be that Harry is cursed in a way that slowly changes his values and beliefs. As he starts to side with Voldemort, he begins to enforce new, dark rules in Hogwarts. The other students are in shock and disbelief. Some of his friends like Hermione might try to break the curse, while others like Ron might be more quick to condemn him. Also, Voldemort could use Harry as a tool to gain more support from the wizarding community, as Harry was once seen as the 'Chosen One' to defeat him. So with Harry on his side, Voldemort could deceive many into thinking his ways are not so bad.
Perhaps Voldemort's son is raised in secret by a group of witches and wizards who want to give him a chance at a normal life. But as he grows older, he starts to show signs of his father's power. This could lead to a conflict within him as he decides whether to embrace or reject his dark powers. He could also go on a journey to learn about his father's past, only to be horrified by what he discovers and then vow to be different.
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.
There might be a fanfiction where Voldemort discovers Snape's true loyalty is not fully with him. This could lead to a really tense showdown between them. Snape would have to use all his skills in magic and deception to survive. Maybe he has to convince Voldemort that he was only pretending to be against him to gain more information for the Dark Lord.
One idea could be that Harry discovers a new spell that can penetrate Voldemort's dark magic defenses and uses it to shoot him. Another could be that Harry teams up with some unexpected allies, like the house - elves, who help him get in a position to shoot Voldemort. Maybe Harry has to sacrifice something important, like his invisibility cloak, to gain the power needed to fire the shot that will defeat Voldemort.
Perhaps there's a fanfiction where Hermione disguises herself as one of Voldemort's supporters to infiltrate their ranks. She has to be extremely careful not to be discovered as she tries to find a way to stop a major plot they are hatching. Along the way, she discovers the complex reasons some people became supporters and uses that knowledge to her advantage.