Harry might be initially skeptical. After all the years of torment from Dudley, he can't just immediately trust this change. But if he sees that Dudley is truly sincere, he could accept the apology with grace.
One possible reason could be that Dudley has finally grown up and realized how mean he has been to Harry all these years. Maybe he had an experience that made him see the error of his ways, like facing real danger himself and seeing Harry's bravery in a new light.
One possible plot could be that Harry plans a very elaborate and private proposal. He takes Ginny to their favorite place in Hogwarts, perhaps the Room of Requirement set up as a beautiful garden filled with the flowers Ginny loves. He gets down on one knee and tells her how she has been his rock through all the post - war chaos and how he can't imagine a future without her.
Well, perhaps Dudley uses his street - smart knowledge. He might teach Harry some rough - and - tumble moves he's learned from his own scuffles in the neighborhood. For example, showing Harry how to dodge a punch quickly.
Harry might become more open and less guarded. Since he now has a brother in Dudley, he might feel more at home in the Muggle world. Dudley, on the other hand, could become less of a bully. The knowledge that he has a wizard for a brother could make him more humble and empathetic.
I'm not sure if there is a specific 'Dudley Gets Harry mpreg fanfiction'. Fanfiction covers a vast range of ideas and pairings, but this one seems rather unusual. It might exist in the extremely diverse world of fan-created works, but it's not a common or widely - known type of fanfiction. You could try searching on specialized fanfiction websites to find out for sure.
Maybe Dudley sees a different side of Harry through fanfiction. He might be curious about the magical world from Harry's perspective that he didn't fully understand before. Fanfiction often explores characters in new and interesting ways that could intrigue Dudley.
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.
Well, in a lot of fanfictions, forgiveness is a big theme. If the writer wants to explore the idea of redemption or growth, Harry could very well forgive Griffindor. However, if the fanfiction is more about conflict and drama, Harry might hold a grudge. There are also cases where Griffindor might not even do something that requires forgiveness in the first place in a particular fanfic.