Another idea is that Snape faked his own disappearance. He might have a mission that requires him to be 'missing'. He could be secretly working on a plan to take down Voldemort once and for all. He might be gathering allies or information while everyone thinks he's vanished.
One possibility could be that he has gone into hiding to protect a secret. Maybe he discovered some dark magic plot and has to keep himself away from the wrong people until he can figure out how to stop it.
Perhaps he has been kidnapped by a group of dark wizards who want to use his knowledge of potions and the Dark Arts. They might be planning something big and need Snape's expertise. He could be held in an unknown location, fighting to escape.
If Snape goes missing, Hogwarts would be in chaos. His students would be worried and confused. The other teachers would have to take over his classes, but no one could replace his unique teaching style.
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.
A possible plot could involve Hermione inheriting Snape's potions skills in an even more profound way. She could use her combined intelligence from her mother and potions talent from Snape to create new spells or potions. Snape, as her father, would be torn between his loyalty to the Dark Lord (in some versions of the story) and protecting his daughter from the chaos of the wizarding war.
One idea could be that Snape is secretly asked to dance by a student who sees through his tough exterior. Maybe they share a moment on the dance floor where Snape reveals a softer side, reminiscing about his own Hogwarts days as a student during the Yule Ball. This could lead to a new understanding between the student and Snape.
One idea could be that the baby has Snape's intelligence and Lily's kind heart. In the fanfiction, perhaps the baby grows up in a world where the truth about Snape's love for Lily is known. This could lead to a lot of interesting interactions with other characters from the Harry Potter universe. The baby might be a powerful wizard or witch, inheriting both Snape's potions skills and Lily's natural magical talent.
One idea could be that Snape takes Harry in secretly. He teaches Harry advanced potions and magic in a more personal way, away from the prying eyes of Hogwarts. Harry begins to see Snape in a new light, not as the mean professor but as a strict yet caring guardian.
One idea could be that Hermione accidentally travels back in time and meets a younger Severus Snape. She could initially be shocked by his younger appearance and different demeanor. Maybe she tries to help him with his studies or deal with the bullies in his life, which would create an interesting dynamic as she already knows his future self.
One idea could be that in the fanfiction, the collar is a magical device used for protection rather than a form of control. Snape might put it on Harry to shield him from dark magic threats at Hogwarts.
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.
A possible fanfic could explore what would happen if Severus Snape had to save James Potter's life. Maybe James is in a life - threatening situation during a battle with Death Eaters. Snape, despite his hatred for James in some ways, decides to help him because he realizes that James is important to Lily. This event could be a turning point in their relationship, making Snape question his own feelings and James to reevaluate his view of Snape.