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 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.
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.
One way Marge can help is by being a plot device. Her presence can force the wizard characters to be more cautious about using magic, which adds tension to the story. For instance, if they are in a situation where Marge is nearby, they have to hold back their magical abilities, and this can lead to some interesting and suspenseful scenarios.
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.
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 fanfiction, if Aunt Marge found out Harry was a wizard, she might have an extreme reaction. She could be terrified and try to distance herself from him immediately. Maybe she would even call the authorities in the Muggle world, not understanding that they can't do anything about wizardry.
Maybe she has a secret soft spot for animals, like the dogs she breeds. In a fanfiction, she could find out that some magical creatures are being mistreated, and she decides to use her influence in the Muggle world to help protect them. This would show a different side of her and make her more likable.