I'm not sure if there is a specific fanfiction with that exact plot. Fanfiction is a vast world, and while there might be some out there that explore unique and unexpected scenarios between Harry and Snape, this one seems rather unusual. You could try searching on popular fanfiction websites like Archive of Our Own or Fanfiction.net.
Harry could be following a hunch. He might have found some evidence that links Snape to a solution he needs. For example, he could have discovered a note written by his father or mother that mentions Snape's ability to deal with a certain type of dark magic that is now threatening Harry. So, he takes a chance and goes to Snape, hoping against hope that Snape will be willing to help.
The most obvious consequence would be that Voldemort would likely betray Harry. Voldemort is evil at his core, and he would probably use this opportunity to harm Harry or gain more power. For example, he could steal Harry's magic or use him as a pawn in a new scheme.
Snape's help could have a huge impact on Harry's Hogwarts journey. For example, he might give Harry tips on dealing with certain dark magic threats secretly. Since Snape is a very knowledgeable wizard, his insights into the history of Hogwarts and the secrets of magic could help Harry better understand the school's mysteries and use them to his advantage.
In some fanfictions, Snape might use his advanced knowledge of potions to create a special concoction to heal Harry. He could draw on his skills in Herbology as well to find rare ingredients for the potion.
In this kind of fanfiction scenario, Snape might initially be conflicted. On one hand, he has his long - standing dislike for James Potter, Harry's father. But on the other, his sense of duty as a Hogwarts professor could kick in. He might reluctantly start to heal Harry, all the while muttering under his breath about how much trouble Harry always seems to get into, just like his father. Then, as he heals Harry, he might start to see some of Lily, Harry's mother, in him and soften a bit.
One more to consider is 'Snape: Beyond the Surface'. In this story, the author explores Snape's relationships with other characters like Dumbledore and McGonagall. It shows how his interactions with them were not always as straightforward as they seemed in the original books and adds more depth to his character.
One great Snape fanfiction is 'The Prince's Tale Redux'. It delves deeper into Snape's relationship with Lily. It shows his internal struggles and how his love for her influenced his actions throughout the series. It gives a more in - depth look at his character than what was shown in the original books.
In some fanfictions, Snape is indeed portrayed as Harry Potter's father. Fanfiction allows for creative reinterpretations of the original story, and this is one such interesting deviation from the canon where Snape's complex relationship with Harry takes on a whole new dimension.
In the world of Harry Potter fanfiction, Harry might need help in various ways. For example, he could need help dealing with a new dark wizard threat that the original books didn't cover. Maybe he's struggling with his post - war trauma and needs the support of his friends more than ever. Another possibility is that he's in a situation where he has to learn a new and difficult magic spell to save someone he loves, but he can't seem to master it on his own.