One idea could be an Erza Scarlet crossover with the world of Harry Potter. Erza could be a visiting wizard from another magical realm. She might attend Hogwarts for a while, and her combat skills could be put to use in the Triwizard Tournament, adding a new and exciting dynamic to the traditional challenges.
Another option is a crossover with the world of Fullmetal Alchemist. Erza could end up in Amestris, and her unique abilities and sense of justice would lead her to get involved in the fight against the Homunculi. She might also be intrigued by the concept of alchemy and try to find ways to incorporate it into her own fighting style.
Another plot could involve a crossover with the Harry Potter world. Erza could be a transfer student to Hogwarts. Her magic, which is different from the wizarding magic, would create a stir. She might have to face off against Voldemort's forces while also learning about the magical world of wands and spells.
One idea could be a story where they accidentally swap bodies. This would lead to a lot of comical situations as they try to adapt to each other's lives. They'd have to deal with different relationships within their respective guilds or groups while also learning more about each other's personalities.
One plot idea could be that a group of dark mages kidnap Erza to use her as a bargaining chip against her guild. They know her strength and reputation, so they think they can force Fairy Tail to give up some valuable magical artifacts. Another could be that an obsessed fan of Erza's kidnaps her, believing that she can be 'tamed' and made to love him. This could lead to a story of Erza having to outwit this deranged individual while also trying to escape from his clutches.
One idea could be exploring how Erza balances her pregnancy with her duties as a strong mage. Maybe she has to take a break from dangerous missions and feels a bit restless. Another could be the reactions of her friends in Fairy Tail. They might be overly protective or excited for her.
A cool plot could be that there's a magical artifact that has the power to grant any wish. Ichigo and Erza both hear about it independently and end up chasing it. When they meet, they initially think the other is an enemy. But as they start to understand each other's motives, they decide to share the wish if they get the artifact. However, they face many challenges along the way, like traps set by other people who also want the artifact. Erza's strategic mind and Ichigo's determination would be great assets in this adventure.
One idea could be that Naruto and Erza are brought together by a powerful magic spell that transports Naruto to the Fairy Tail world. They initially misunderstand each other due to their different backgrounds and abilities. But as they face common enemies, they start to learn from one another. Naruto could teach Erza about his ninja way of never giving up, and Erza could show Naruto the power of magic and the importance of friendship within the guild.
Well, the plot could involve Naruto growing up in the Fairy Tail world with Erza as his mother. He might inherit some of Erza's strong - willed and determined nature, as well as Naruto's own unique ninja abilities. Maybe he has to face some sort of threat that combines elements from both the Naruto and Fairy Tail universes.
In a crossover with One Piece, Zoro would be a great character to pair with Erza. They both are highly skilled in combat, with Zoro's three - sword style and Erza's multiple weapon - based attacks. Their determination and loyalty to their friends are also similar traits, and they could engage in some epic battles together while also learning from each other's fighting cultures.
One idea could be that they team up to face a new, powerful magical threat that neither could handle alone. Maybe it's a dark sorcerer who is trying to take over the world by manipulating the minds of powerful heroes. They would have to combine Black Widow's combat skills and strategic thinking with Scarlet Witch's reality - warping powers.
A possible plot for a Harry Potter Scarlet Knights fanfic could be centered around a prophecy related to the Scarlet Knights. It could be that a new student arrives at Hogwarts and is revealed to be the key figure in the prophecy. The Scarlet Knights then have to protect this student from Voldemort's followers who also know about the prophecy. Along the way, the student discovers their own hidden powers and the true nature of the Scarlet Knights, leading to a battle between good and evil at Hogwarts.