There could also be interesting developments for Harry Potter in such fanfictions. Harry might have a different reaction to Snape. Instead of the complex and often conflicted relationship they had in the original story, Harry could fully embrace Snape as a hero and perhaps learn more about Snape's past and his motives in a more positive light. This could lead to a much closer bond between them, with Harry seeing Snape as a mentor figure in a new way.
In some fanfictions, Snape might be working with Voldemort because he has been blackmailed. Voldemort could have found out some deep, dark secret about Snape and is forcing him to cooperate. This could be related to Snape's past or something he has done that he doesn't want to be made public.
One possible plotline could be that Snape discovers a hidden vulnerability in Voldemort. Maybe it's related to his Horcruxes. Snape, being the intelligent and resourceful character he is, figures out a way to destroy all of Voldemort's Horcruxes without Voldemort suspecting. Then, he uses his powerful magic skills, perhaps a combination of dark and light magic, to directly confront and kill Voldemort.
One interesting fanfiction could be 'Snape's Redemption: Voldemort's End'. In this fic, Snape uses his deep knowledge of the Dark Arts against Voldemort. He discovers a long - forgotten spell that can counter Voldemort's main source of power. It's a really exciting read as it shows Snape's cunning and his determination to finally put an end to Voldemort's reign of terror.
There's a fanfic where Snape takes advantage of a power struggle within Voldemort's ranks. He poisons Voldemort's drink during a meeting, which causes Voldemort to become severely ill, an injury in a sense. Snape does this to try and disrupt Voldemort's plans and give the Order of the Phoenix more time to prepare their defenses.
Power. Voldemort might see Snape as a means to gain more power. Snape is highly intelligent and skilled in the Dark Arts. By caring for him, Voldemort could be ensuring that Snape remains loyal and continues to use his skills for Voldemort's benefit.
He could survive through his strong will and Occlumency skills. By blocking Voldemort from fully penetrating his mind, he can keep his most important secrets safe and hold on to his sanity, which is crucial for survival.
A common plot is that Snape uses his position as a double agent to his advantage. He gets close to Voldemort and then at a key moment, he uses a hidden weapon or spell to injure him. For example, he might have a wand with special properties that he uses against Voldemort. This is popular because it plays on Snape's duplicitous nature.
One possible plotline could be that Snape discovers a hidden weakness of Voldemort. Maybe through his years of spying, he finds out that a certain spell, when combined with an ancient magic artifact, can destroy Voldemort. So he devises a plan to get hold of the artifact and uses the spell at a crucial moment when Voldemort is off - guard, perhaps during a major battle when Voldemort is focused on other opponents.
One unique aspect is the exploration of Snape's double - agent status. Fanfictions often dig deeper into how he managed to deceive Voldemort for so long. For example, some stories might focus on the spells or mental techniques he used.