A possible plot could be that a wizard from the Harry Potter world accidentally transports himself into the world of Catelyn and Arya. He brings with him a lemon that has been enchanted. This lemon has the power to change the fates of those who consume it. Arya, being the adventurous one, wants to taste the lemon despite Catelyn's warnings. Harry Potter - like elements such as spells and potions start to blend with the medieval - like world of Catelyn and Arya as they deal with the consequences of the enchanted lemon.
Perhaps the lemon is a key element in a prophecy. In this fanfiction, Catelyn has a vision that involves a lemon, Arya, and elements from the Harry Potter universe such as Hogwarts. Arya is on a quest to find out more about this prophecy. She teams up with characters from both worlds, and together they try to decipher the meaning of the lemon in the prophecy. It could be that the lemon represents a source of power that can either save or destroy their world, much like the Horcruxes in Harry Potter.
One idea could be that Arya, while on her travels, comes across a lemon tree in a strange land. She remembers her mother Catelyn's love for the smell of fresh things. So, she decides to take some lemons with her to bring a bit of home. When she finally returns to a place where she can reunite with Catelyn's memory, perhaps through an old family friend, she uses the lemons to make a special drink or offering in her mother's honor.
One possible plotline could be that in a magical crossover world, Harry Potter somehow stumbles into the world of Leah Clearwater. Maybe the lemon aspect could be a magical fruit with special powers that they have to find together. They could face various challenges along the way, like dark wizards who also want the lemon for its power.
In the fanfiction, Arya might see the lemon as an opportunity. Since she's always been resourceful, she could think of ways to use it to her advantage, perhaps in her battles or quests. Catelyn would be more concerned about the implications of this strange object in their world. Harry Potter, with his knowledge of magic, could try to communicate with the lemon if it had some sort of magical essence. He might also try to protect Catelyn and Arya from any potential harm the lemon could cause, while Arya might be more eager to take risks with it.
A possible plotline could be that Harry, after the war, is magically bound to Narcissa for some reason. In the harem setting, other characters like Luna or Ginny could also be part of it. Harry has to figure out how to balance his relationships while also dealing with the post - war wizarding world. Narcissa might use her influence to protect Harry from those who still hold grudges against him.
Arya could use a lemon as a secret signal related to her family. Catelyn taught her this long ago. So when Arya is in a dangerous situation, she leaves a lemon as a sign that she is from House Stark. This way, it becomes a connection between her and her mother's teachings.
One possible plotline could be that Winky Lemon, if she has some unique magical abilities, helps Harry in a difficult situation at Hogwarts. Maybe she has a special connection to a part of the castle or a magical creature that is crucial to solving a mystery.
One possible plotline could be that Apolline Delacour, Fleur's mother, has some sort of encounter with Harry Potter that involves a lemon. Maybe she uses lemon in a potion she's making when Harry comes to visit the Delacour family. And this simple lemon in the potion could lead to some unexpected magical effects that tie Harry and the Delacour family in a new and interesting way.
There are quite a few potential plotlines. For example, imagine that a powerful magical artifact from the Harry Potter universe is stolen by Loki and he uses it to create chaos in both the Avengers' world and the wizarding world. At the same time, a character with the same unpredictable nature as Ledger's Joker emerges in the mix. The Avengers and Harry Potter characters would have to put aside their differences and use their unique abilities. The wizards could use their magic, the Avengers their superpowers, and they'd all have to deal with Loki's trickery and the chaos - causing character, leading to a complex and exciting adventure.
One possible plotline could be that Harry's half - sister is initially on the side of the dark forces. She could be manipulated by Voldemort or his followers. Harry then has to try and save her and turn her to the good side. This would be a classic good - vs - evil struggle within the family.
Perhaps Harry discovers that he has basilisk blood in him. This could change his abilities and how others view him. He might face discrimination from some in the wizarding world who fear the basilisk's power, while others might see him as a potential weapon against Voldemort. The plot could revolve around Harry coming to terms with this new aspect of himself.