Maybe Dumbledore steps in at the last moment and takes Harry away from Vernon's house. He could use his powerful magic to protect Harry and remove him from that hostile environment.
Perhaps some of Harry's school friends, like Hermione and Ron, find out about Harry's plight with Vernon. They could use magic (even though it's against the rules for underage wizards outside of school in normal circumstances) to create a diversion and then spirit Harry away. They might use spells like the Confundus Charm on Vernon to make him confused long enough for them to get Harry out of there. Hermione, being very clever, would probably come up with a well - thought - out plan to save Harry from Vernon.
In the fan fiction, it could be that Hagrid arrives unexpectedly. Hagrid, being a half - giant and very loyal to Harry, might physically overpower Vernon and rescue Harry. He might have been sent by the wizarding world when they learn of Harry's situation with Vernon. For example, Hagrid could break down the door of the Dursleys' house and just carry Harry off to safety, ignoring Vernon's protests. Hagrid has a soft spot for Harry and would do anything to keep him safe. Also, the other magical creatures that Hagrid is friends with might also play a role. Maybe he brings along Fang, his dog, who could scare Vernon away, allowing Hagrid to save Harry.
It could be Sirius Black. Sirius is Harry's godfather and he is very protective of Harry. He might break out of his confinement (if he is still in that situation in the fan fiction) and rush to Vernon's house. Sirius has the skills and the determination to face Vernon and save Harry. He could transform into his Animagus form, a large black dog, and intimidate Vernon into letting Harry go.
Maybe Vernon is jealous of Harry's magical abilities. He might see Harry as a threat to his 'normal' family image. Also, Vernon has always been a bully at heart and Harry, being an easy target in his house, becomes the victim of his pent - up anger.
In this fan fiction scenario, Snape might use his knowledge of the Muggle world and Vernon's weaknesses. Snape could have found out some information about Vernon's business affairs or personal secrets. Then he blackmails Vernon into leaving Harry alone. By doing this, he effectively saves Harry from Vernon without having to use too much magic and draw unwanted attention.
Vernon might be jealous of Harry's magical abilities. He could see Harry as a threat to his 'normal' family life and his authority in the household.
Petunia could be a great addition. Her relationship with both Harry and Vernon adds a lot of complexity. She could be the mediator or the one causing more trouble between them, depending on the plot.
In this kind of fanfiction, perhaps Vernon is driven by jealousy. He sees how much attention Harry gets, even if it's often negative attention within the household. This jealousy festers until he decides to take drastic action. It could also be related to some long - held family secrets or grudges that are finally brought to the surface, leading Vernon to try and kill Harry.
In some fanfictions, Harry might cope by finding solace in his friends at Hogwarts. He could talk to Hermione and Ron about what Vernon does, and their support gives him strength. For example, they might encourage him to focus on his magic studies as an escape from the Dursley's household.
One common way is through the power of friendship. Harry's friends, like Ron and Hermione, might be vigilant and notice the stalker's actions. They could then use their combined magic and wits to outsmart the stalker and save Harry.
It could be that Snape wants to spite Vernon. Vernon represents everything that Snape despises about Muggles who are cruel and unaccepting. By saving Harry from Vernon, Snape is showing his superiority over Vernon and also his power as a wizard. And of course, there's still that underlying reason of his love for Lily which makes him want to protect her son.
If Snape witnesses Vernon abusing Harry in fan fiction, he might use it as an opportunity to gain some leverage over Harry. Snape could offer to keep quiet about the abuse in exchange for Harry doing certain things for him, like getting good grades in Potions. However, this is a rather cynical view and goes against Snape's more complex character who deep down has some good in him.