A possible plot could be that Petunia was cursed as a child which made her seem non - magical, but as she grows older, the curse begins to fade. She then has to learn how to control her newly discovered powers while also dealing with the fact that the wizarding world she once despised now needs her help. She could become an important ally to Harry and his friends, using her unique perspective as a Muggle - raised witch.
The plot could involve Petunia using her witch powers to protect Harry from the Dursleys' abuse. She realizes that she has a responsibility towards her sister's son and uses her magic to create a safe haven for him within the Dursley household. This would be a big change from her original character who was so against magic.
Perhaps Petunia discovers that she has the power to travel between the Muggle and wizarding worlds at will. She then embarks on a journey to learn more about her family's magical history and in the process, uncovers secrets that have been long buried. This could also lead to her helping Harry in his fight against Voldemort in unexpected ways.
One possibility is that Petunia could use her witch powers to protect Harry secretly. Maybe she has a change of heart after years of resentment. She could have been in contact with the wizarding world all along, but hid it from everyone. This would add a whole new layer of mystery and family drama to the Harry Potter universe.
If Petunia was a witch, it would completely change Harry's upbringing. He might have learned magic from her instead of being in the dark until he received his Hogwarts letter. This would make him more confident and perhaps less of an outcast in the Muggle world.
One possible fanfiction could be about Petunia discovering her latent witch powers later in life. Maybe she stumbles upon an old family heirloom that unlocks her magical abilities. This could lead to a whole new dynamic in her relationship with Harry, as she now understands the world he belongs to on a deeper level.
She could be on a quest to clear her father's name. Since Sirius was wrongly accused of many things, she might dig into the past, find evidence that he was innocent all along. For instance, she could discover hidden letters or witnesses that were overlooked. This would lead her into many dangerous situations as there are those who would rather keep the truth buried.
Perhaps Fleur Delacour's sister is an exchange student from Beauxbatons to Hogwarts for a special program. She arrives with high expectations but is met with some resistance from the Hogwarts students who are a bit jealous of her connection to Fleur. However, she quickly proves herself by being extremely talented in a different area of magic, like potion - making. She then enters the Triwizard Tournament as an honorary contestant (due to some special rule) and has to face various challenges, all while trying to make friends and fit in at Hogwarts.
Perhaps Aunt Petunia is a witch who was cursed long ago. The curse made her forget her magic and act like a Muggle. When Harry unknowingly breaks the curse, she suddenly has her powers back. She then has to face the wizarding world again and deal with all the changes that have occurred since she last remembered her true self. She might even have some unique powers that are different from the norm because of the curse.
The plot could start with Petunia having a strange encounter with an angel. The angel seems to know things about her past and her relationship with Lily. At the same time, an elf, which has been observing Petunia for some reason, decides to approach her. The angel wants to help Petunia understand the importance of magic and her family's place in it. The elf provides some practical help, like getting access to some magical items. As the story progresses, Petunia changes her view of the wizarding world, and together with the angel and the elf, they might even influence some of the events in the wizarding world, like helping to protect Hogwarts or something similar.
One possible plot could be that Petunia is somehow magically enslaved by a dark wizard. Harry, upon finding out, feels a sense of obligation to save her. He might have to use his knowledge of magic and his connections at Hogwarts to figure out a way to break the spell. Along the way, he could discover some secrets about Petunia's past that were related to the wizarding world.
One possible fanfiction could be about Aunt Petunia discovering her latent witch powers late in life. Maybe she starts having strange dreams that lead her to an old family heirloom that unlocks her magical abilities. She then has to deal with the shock of it all while also trying to keep it a secret from the Muggle world around her.
Lemon could be involved in a mystery at Hogwarts. For instance, she might notice some strange happenings in the restricted section of the library. Since house - elves can move around unnoticed, she starts to investigate. As she delves deeper, she realizes it has something to do with an old, forgotten magic that was once banned in the wizarding world.