One possible plotline could be that Harry has done something extremely dangerous or against the wizarding laws in the fanfic world. Lily, as his mother, feels that she needs to discipline him in a more severe way than just a talking - to. So she resorts to spanking as a last - ditch effort to make him understand the gravity of his actions.
If such a fanfic were to exist, perhaps it could start with Harry doing something extremely reckless that Sirius feels compelled to discipline him in this extreme way. Maybe Harry has endangered himself or others in a way that Sirius believes requires a strong reaction. But this would be a very different take on their relationship compared to the original books.
Yes, there might be some fanfics with this concept that are written in a non - offensive and family - friendly way. However, spanking is a rather controversial topic even in a fictional context. Some fanfic authors might handle it in a way that focuses on discipline in a more positive light, while others may not.
One possible plotline could be that a dark wizard has somehow managed to subjugate some Gryffindor students and is using them for his own dark purposes, like extracting magic or performing dangerous tasks. The other Gryffindor students, along with students from other houses, would then have to come together to rescue their enslaved comrades. This would create a story of resistance and heroism against an unjust situation.
Well, one plotline could be that Lily uses her influence to change the way Hogwarts is run. Since she was a brilliant witch, she might introduce new spells and teaching methods. For example, she could start a special class for students to learn advanced defensive magic earlier.
One possible plotline could be Harry coming to power after a great war in the wizarding world. He then has to deal with rebuilding and unifying the different wizarding communities under his rule as emperor. He might face resistance from some traditional wizarding families who don't want to accept a new form of government.
In a 'Harry Potter is an assassin' fanfic, the plot could start with Harry being trained by an unknown and powerful wizard who has his own motives for creating an assassin out of Harry. This trainer could be someone from a far - off land or an ancient order. Harry then starts taking on targets within the wizarding world that are supposedly 'evil' but as he delves deeper, he realizes that things are not as black and white as he was told. His friends start to notice his strange behavior and he has to either come clean or keep lying to them. As the story progresses, Harry might have to make some tough choices between his loyalty to his friends and his new - found assassin duties, all while uncovering secrets about the wizarding world that were previously hidden.
One possible plotline could be that Lily, now as a girl, has to navigate the wizarding world all over again. She might face different challenges and opportunities than she did before. For example, she could form new friendships or rivalries at Hogwarts.
One possible plotline could be that Lily and James return during the time of Harry's battle with Voldemort. Their return could give Harry new strength and hope. They might use their old magical knowledge and skills to help in the fight. Maybe they could also reunite with their old friends like Sirius and Remus, and together they form a powerful force against the Dark Lord.
Well, one possible plot could be that Xehanort's essence or some form of his power gets transferred into the Harry Potter world. This power could start to corrupt some of the wizarding community, and Harry, with his strong sense of right and wrong, has to trace the source of this new evil. Along the way, he discovers more about Xehanort's past and motives, and has to use his knowledge of both the wizarding world and Xehanort's methods to defeat this new threat. It would be a complex plot that would require a lot of world - building to combine the two very different fictional settings.
I'm not aware of any such fanfic that would be considered appropriate in a general sense. Spanking is a rather inappropriate and unwholesome concept to be associated with the characters in this way. The Harry Potter series is mainly about magic, friendship, and fighting against evil, and this kind of fan - created content goes against the positive themes of the original work.