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.
Well, in such a fanfiction, it could greatly change the storyline. Snape is often portrayed as a complex and somewhat bitter character in the original story. But if he takes care of baby Harry, he might develop a different kind of bond with him. Maybe he'd become more protective and less resentful towards Harry as he watches him grow. This could lead to a completely different dynamic between Snape and the other characters too, like Dumbledore. He might not be as secretive from Dumbledore about his feelings for Harry.
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.
In the fanfiction, Harry's relationship with Snape himself would of course be very different. When Snape grows up, he might have a different view of Harry. Since Harry took care of him as a baby, there could be an underlying sense of gratitude, although Snape might not show it openly. And Harry, having seen Snape as a vulnerable baby, might have more empathy for him in their later interactions, which could change the whole dynamic of their relationship compared to what we see in the original books.
Maybe it's the child of one of his friends or allies. For example, it could be the baby of Ron and Hermione. Harry would then be taking care of the baby out of loyalty and friendship.
It's popular because it creates an interesting dynamic. Snape has always had a complex relationship with Harry in the original series. When Harry is deaged, it gives Snape a chance to start anew and perhaps be a different kind of influence. It also allows for exploring Snape's more nurturing side which is a contrast to his usual cold persona.
Harry might first try to figure out what's wrong. If the baby is hungry, he'll quickly get a bottle. If it's a wet diaper, he'll change it. Sometimes, he just holds the baby close and rocks it gently, humming a little tune. He doesn't get frustrated easily because he really cares about the baby.
A heartwarming moment could be when Harry takes the baby out for a walk. He pushes the stroller carefully, talking to the baby about all the things they see around them, like the colorful flowers and the friendly squirrels. And at night, when the baby has a nightmare and Harry stays up all night comforting it. He rocks the baby gently and whispers soothing words until the baby calms down and falls back asleep.
In this fanfiction, perhaps Harry could struggle at first with the basic tasks like feeding and changing the baby. He might make mistakes but learn quickly.
One possibility could be that Draco is forced into taking care of Harry due to some magical circumstances. Maybe there's a spell gone wrong that makes it his responsibility. He initially resents it but as he spends time with baby Harry, he starts to see the innocence in him and develops a soft spot.