In some fanfictions, Harry could react in a very calculated way. He might start to plot a way to get back at Aunt Marge without using magic in an obvious way. He'd use his wits and the knowledge he has of her weaknesses. For example, if he knows she's very proud of her dogs, he could find a way to make her look foolish in front of others when it comes to her precious pets. All this while still seething about the insults to his parents.
Aunt Marge could be developed by showing her having a moment of self - reflection. She might realize that her harsh treatment of Harry was wrong and start to be more empathetic. For example, she could see Harry's kindness towards animals and that could change her view of him.
One consequence could be that she gets in trouble with the wizarding world. If Harry reports her to the proper authorities, like the Ministry of Magic, they might take action against her for using such offensive language towards the parents of a young wizard. They could issue a warning or even a small fine.
In the fanfiction, they could be forced to work together on a project related to lemons. Maybe there's a local fair that requires everyone to contribute something lemon - themed. Through this shared task, they begin to understand each other better. Harry realizes that Aunt Marge has her own insecurities and Aunt Marge sees that Harry is not the troublemaker she thought he was. For instance, Harry could show his creativity and intelligence during the project, impressing Aunt Marge.
It could develop into a more tolerable one. Harry might start to see Aunt Marge as a pitiful figure rather than just a mean old woman. Maybe he discovers some things about her past that make him more empathetic.
There could be a 'Aunt Marge likes Harry fanfic'. Fanfic communities are full of diverse interpretations of characters and relationships. It might explore a different side of Aunt Marge's character, perhaps in an alternate universe where she doesn't hold her usual grudges against Harry. You could also try reaching out to Harry Potter fanfic communities on social media platforms to ask if anyone has read or written such a fanfic.
It can also influence new fan creations. Some new fanfiction writers might be inspired by the more controversial 'Harry Potter Insults Fanfiction' to either create their own version of it or to create works that are a reaction against it. For instance, they might write stories that focus on healing the relationships that are damaged in the insults fanfiction.
Harry could be angry at first. He might think his parents are overreacting or don't understand his situation at Hogwarts. But then, after thinking about it, he realizes they are just worried about him and starts to feel guilty.
One possible plot could be that Aunt Marge discovers Harry's difficult situation at the Dursleys and decides to secretly help him with his magic studies. Maybe she has a change of heart after seeing how much he's struggling.
He might be extremely angry and confused. After all, parents are supposed to be the ones you can trust the most. He could start to question everything he knew about his family and himself.
Harry would probably be overjoyed at first. After all, he has always longed for his parents. He might start to look for ways to make it happen, like searching for magic spells or ancient artifacts that could bring his parents back.