Perhaps he goes back in time and has the chance to rewrite his own past. He could choose to not get involved with the Death Eaters in the first place, which would completely change his future self and the events at Hogwarts. He might also try to befriend Harry's father earlier to avoid the animosity that developed between them.
One interesting plotline could be Snape going back and trying to prevent the death of Lily Potter. He might use his knowledge of the future to change certain events that led to her demise.
Another plotline could be Snape going back and influencing the development of magic at Hogwarts. He could introduce new spells or potions that he learned in his original timeline. He might also try to change the way the houses are sorted or the relationships between different magical creatures at the school.
Bellatrix Lestrange could also be a character who interacts with Snape. If Snape goes back far enough, he might try to influence Bellatrix's path and keep her from becoming such a devoted Death Eater. This could have a major impact on Voldemort's power and the overall balance of the wizarding world.
One interesting plot could be Severus going back to his school days and trying to prevent certain tragedies. For example, he might try to stop the rift between him and Lily. He could use his knowledge of the future to avoid making the mistakes that led to their falling out. Another plot could involve him changing the course of events related to Voldemort's rise. Maybe he tries to expose Voldemort's plans earlier or influence key characters to oppose him more effectively. He could also go back to a time before he got involved with the Death Eaters and choose a different path entirely, which would then have a huge impact on the wizarding world as we know it.
One interesting plot could be Snape going back and trying to prevent the death of Lily. He might use his knowledge of the future to change certain events that led to her demise. Maybe he would try to expose Pettigrew earlier or find a way to make James Potter more cautious.
Perhaps a plot where Severus discovers his demigod heritage and has to balance it with his life at Hogwarts. He could struggle with how to use his new - found powers without drawing too much attention, while also dealing with his complex relationships with other characters like Harry Potter.
One interesting plotline could be exploring Snape's early interactions with the Marauders. Maybe it shows how his enmity with them truly began. For example, it could focus on a particular incident in their school days that was a turning point.
One interesting plotline could be the protagonist using Snape's knowledge of potions to create new and revolutionary brews. For example, they might discover a potion that can heal previously incurable dark magic wounds. Another plotline could involve the character trying to change Snape's fate in the wizarding world. They could attempt to prevent Snape from getting involved in certain dangerous or tragic events, like his involvement with the Death Eaters in his youth. Also, there could be a plot where the reincarnated character uses Snape's occlumency skills to uncover hidden secrets in Hogwarts that were previously unknown.
In this type of fanfiction, a great plotline could be Sansa influencing the political landscape much earlier. She could use her knowledge of how certain houses would act in the future to her advantage. For example, she could form early alliances with houses that she knows will be important later. She might also try to change the way her family views certain people. Maybe she could convince her father to see Littlefinger's true nature much sooner. This would not only protect her family but also change the entire power dynamic in Westeros.
One interesting plotline could be Castiel going back in time to prevent a major disaster in the past that has far - reaching consequences for the present. For example, he might stop a powerful demon from making a deal that would later lead to the death of many hunters.
In some 'Vegeta goes back in time' fanfictions, Vegeta might go back to the time when the Saiyans were still under the rule of King Vegeta. He could try to change the Saiyan culture from being so war - like and brutal. He might introduce new ways of thinking and training that would not only make the Saiyans stronger but also more united. This could lead to a completely different future for the Saiyans, where they don't get wiped out by Frieza and instead become a more dominant force in the universe. He could also form different alliances with other races much earlier, which would change the power dynamics in the galaxy.
Perhaps he goes back in time and decides to train himself better. He could focus on mastering his emotions from the start, which might change the entire course of the Star Wars saga. He might also try to save his mother in a more effective way, without succumbing to anger and hatred.