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.
There are a lot of Harry Potter fanfictions out there. Regarding the ones with Angel, Petunia and Elf, I'm not sure if there's a super - popular one. But you could start by looking at some of the bigger fanfiction platforms. Search for tags like 'Harry Potter', 'Angel', 'Petunia' and 'Elf'. You might find some creative stories where the angel is a magical being from another realm, Petunia has a change of heart due to the angel's influence, and the elf plays a role in the magical happenings around them.
A 'Harry Potter is an elf fanfiction' could have a plot where Harry's elven side is awakened. He starts to have an affinity for nature and elven magic. This newfound power attracts the attention of both good and bad forces. The good forces might want to help him control and use it for the greater good, while the bad forces, like Voldemort, see it as a threat. Harry then has to learn about his elven heritage, the history of elves in the wizarding world, and how to use his new powers to defeat Voldemort. He might also have to deal with the fact that some elves in the wizarding world see him as an outsider, as he was raised as a human wizard, not an elf.
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.
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.
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.
In a more comical plot, the lemon could be a mischievous magical object that keeps causing chaos in Petunia's house. It might make things float or turn into strange creatures. Petunia, being the uptight person she is, has to deal with this lemon - induced mayhem while also trying to keep up appearances in front of the neighbors. This could lead to some really funny situations and a change in her attitude towards the strange and magical over time.
The plot could revolve around Fem Harry Potter being an outcast because of her angelic qualities. The wizarding world might be skeptical or even hostile towards her at first. However, as she starts to show her good intentions and uses her powers to fight against the emerging dark forces, like Voldemort or other dark wizards, she gradually gains acceptance. There could be a love story involved too, where a character falls in love with her not just for her powers but for her kind heart that is a result of her angelic essence.
Perhaps the plot could involve Petunia having some sort of magical awakening or discovery that leads her to perform this transformation on Harry. In this fanfiction, Harry might then use her new female identity to infiltrate parts of the wizarding world that are more difficult for a male to access, like certain all - female magical groups. And this could lead to all sorts of adventures and revelations about the wizarding world's gender dynamics.
One possible plot could be that Harry shows a special talent early on that Vernon and Petunia recognize as something extraordinary. This makes them see Harry not as a burden but as a valuable part of the family. They then start to love and protect him, and this changes Harry's life completely. He no longer has to endure the abuse and neglect he faced in the original story.
I'm not entirely sure as 'lemon' often implies adult content in fanfiction, but if we assume it's a non - adult take, perhaps Harry could dominate Petunia in terms of magical knowledge. Maybe he shows her the true power of magic and she has to accept his superiority in that regard.