Dudley might face his father's anger. Vernon could be furious that Dudley has gone against him to protect Harry.
Dudley's actions could have far - reaching consequences. In the short term, there will be a huge rift in the family. Vernon may try to punish Dudley in various ways, like restricting his privileges. But in the long run, Dudley may inspire others in the family or the community to stand up against unfairness. His relationship with Harry could be a turning point for both of them. Harry could introduce Dudley to the wizarding world in some way, or at least share some of the magical things he has experienced, which could open up a whole new world of possibilities for Dudley.
Perhaps Dudley finally stands up to his father's cruel treatment of Harry. Maybe he realizes that Vernon's behavior towards Harry is wrong and uses his own physical strength or some clever ruse to get Harry out of a difficult situation Vernon has put him in.
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.
In this type of fanfiction, Snape could simply intimidate Vernon. Snape has an air of mystery and a certain power about him. He might show up at Vernon's doorstep, give Vernon a cold stare, and with his reputation as a powerful wizard, Vernon might be too scared to oppose him. Snape could then just take Harry without much resistance from Vernon.
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.
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.
The main characters are Dudley and Harry. Dudley, who has usually been portrayed as a bully in the original series, is now the hero. Harry, on the other hand, is the target of whatever danger is present in the fanfiction. There might also be other supporting characters like Dudley's parents or some of Harry's friends from Hogwarts who could play a role in the story.
It could be that Dudley has his own reasons to oppose the dark forces that are after Harry. He might see that if Harry fails, it could also mean trouble for him and his family. So, he decides to train Harry to fight as a way of self - preservation as well as helping Harry. Another reason could be that Dudley has grown to respect Harry over time and wants to contribute to Harry's ability to defend himself against the many threats he faces.
Another idea for a 'Harry Saves Vernon' fanfic could be set during a time when the wizarding world is under attack and the protection spells around the Dursleys' house fail. Harry, who has a sense of duty towards his aunt and uncle despite their past treatment of him, comes back to save Vernon from the invading dark forces. He might use his Patronus charm to drive away Dementors or use defensive spells to fend off Death Eaters who are targeting the Muggles in Little Whinging.
Maybe Voldemort uses his magic to stun Vernon first. Then he could simply apparate with Harry to a safe place.
Dudley could become more interested in the magical world. He might start to learn more about magic, spells, and the wizarding community, which could open up new perspectives for him.