There might be a fanfiction where Harry takes care of Snape after a battle. He could be the one to tend to Snape's wounds, both physical and emotional. Since Snape has always been a rather closed - off character, Harry's attempts to get through to him and provide care could be a great source of character development for both of them. Harry might use his Gryffindor courage to break down Snape's walls.
In some fanfictions, Harry could take care of Snape in a post - war scenario. Snape has lost a lot during the war, and Harry, feeling guilty for misjudging him for so long, decides to take care of him. Harry could help Snape rebuild his life, find a new purpose. He might introduce Snape to new friends or help him come to terms with his past actions and find peace within himself.
There's 'Snape and the Deaged Savior'. In this fanfiction, Snape discovers deaged Harry in an unexpected place. He takes on the role of caretaker with his usual grumpy exterior but deep down shows a lot of care. He teaches Harry magic in a different way than the normal Hogwarts curriculum, which makes for an interesting read.
One common theme is redemption. Snape is often seen as a character in need of redemption in the original series, and taking care of Harry can be a way for him to achieve that. Another theme is the exploration of family. It shows a different kind of family dynamic than the traditional ones in the Harry Potter world. And there's also the theme of sacrifice. Snape might have to sacrifice his own interests or safety to protect Harry.
You can try looking on Archive of Our Own (AO3). It has a vast collection of Harry Potter fanfictions, and it's likely that you'll find several stories about Harry taking care of Snape there. Just use the search function and type in relevant keywords like 'Harry takes care of Snape' and you should get a list of relevant fics.
A common plot element is Snape's initial reluctance. He might be forced by Dumbledore or some other situation to take care of Harry. But as he does, he starts to see Harry in a different light. For example, in many stories, he realizes that Harry is just a vulnerable boy, not just James Potter's son.
One common scenario is Snape protecting Harry from the Dark Arts attacks within Hogwarts. Another could be Snape helping Harry with his studies privately, especially in potions where Snape is an expert.
One popular fanfiction in this theme might be 'A Dark Path'. In it, Snape's actions towards Harry are complex and seem to border on taking advantage, but as the story unfolds, there are deeper motives at play. Another could be 'Twisted Intentions' where Snape's behavior is initially misconstrued as taking advantage, but it's later revealed to be part of a grander plan to protect Harry.
One interesting plot could be that Hermione discovers Snape is ill or injured in a post - war scenario. She uses her vast knowledge of magic to nurse him back to health. During this process, they start to understand each other better. Snape, who has always been a rather aloof and misunderstood character, begins to open up to Hermione, sharing his past traumas and the reasons behind his actions in the war.
There could be a situation where Harry is being bullied by some students from other houses. Snape, despite his usual cold exterior, steps in and scolds the bullies. For Draco, if he gets into trouble with other Slytherins over some internal house politics, Snape would quietly resolve the issue. In addition, Snape might offer to tutor them personally. He would give Harry extra help with his Patronus charm, knowing its importance, and for Draco, he could teach him some advanced potions techniques that are not taught in regular classes.
Well, at first Harry might be very suspicious. But as Snape shows consistent care, Harry could start to see him in a new light. Maybe he realizes that Snape has more layers than he thought. He might begin to understand that Snape's past actions were more complex than just being mean to him.
One possible plot could be that Harry discovers a time - traveling accident has brought baby Snape to his present. Harry, being the kind - hearted person he is, takes it upon himself to care for the baby. He has to figure out how to feed the baby, keep him clean and comfortable. Maybe he also has to protect baby Snape from any threats in the wizarding world that might target Snape even as a baby.