Harry's magic is mainly based on incantations and the use of a wand. Sakura's magic, especially as Dark Sakura, is more instinctual and connected to her very being. Their powers could interact in a way that Harry's spells could act as a sort of catalyst for Sakura's power. For example, if Harry casts a spell of protection around them, it could enhance Sakura's ability to focus her dark magic without it going out of control.
Another plot could involve a magical tournament that somehow brings Harry Potter and Dark Sakura Matou together. They are forced to compete against each other at first but then realize that there is a greater conspiracy behind the tournament. They then join forces, with Sakura's dark magic and Harry's quick thinking and diverse spellcasting abilities becoming a formidable combination to uncover the truth and defeat the real villains behind the scenes.
One possible plot could be that Dark Sakura somehow gets transported to the Harry Potter universe. Maybe she arrives at Hogwarts and is initially seen as a threat due to her dark powers. But then, she could team up with Harry Potter and his friends to fight a common enemy, like a new dark wizard who is more powerful than Voldemort. Her dark powers could be used in a way that is beneficial in the fight.
One of the most interesting interactions could be between Dark Sakura and Snape. Snape, with his complex personality and his own connection to dark magic, would have a very different view of Dark Sakura compared to others. He might be able to recognize the struggle she has with her dark powers. Dark Sakura, in turn, could see Snape as someone who has walked a similar path. Also, the Weasley twins interacting with Dark Sakura would be a riot. They would be curious about her powers and try to use them for some of their pranks in a harmless way.
I'm not sure specifically about 'Naruto and Sakura Matou lemon fanfiction' as 'Matou' is from a different series (Fate/stay night) and mixing it like this might be a very niche or even non - existent concept in a proper fanfiction context. There could be some very strange crossovers out there, but it's not a common or widely - known combination.
Since this is a very odd combination, it's hard to say exactly. But perhaps it would involve some sort of strange crossover where the worlds of Naruto and Fate/stay night collide, and Naruto and Sakura get involved in a situation related to the Matou family in a romantic or sexually - tinged way, though this is really stretching the boundaries of normal fanfiction concepts.
They are interesting because they offer a new take on the relationship between Naruto and Sakura. It allows fans to explore different scenarios that might not have been possible in the original story.
A common theme is the discovery of new magic. Since Dark Elves have their own magic system, when combined with Harry Potter's world, it often leads to the exploration of new magical abilities. Another theme is friendship. Despite their differences, the characters form bonds.
One possible plot could be that a magical accident transports Sakura into the Harry Potter universe. She could use her ninja skills, like her chakra control, to help Harry and his friends in a new kind of battle. For example, in a fight against dark wizards, Sakura's medical ninjutsu could be used to heal the injured wizards quickly, which is something not common in the wizarding world. And her physical combat skills might also come in handy when they face opponents up close.
Some fanfictions might focus on an alternate universe where Naruto and Sakura Matou have different abilities or roles. For example, Sakura Matou could have a unique power related to her family's secrets, and Naruto has to help her control it while also dealing with threats from those who want to use her power for evil.