Perhaps in the crossover, the Prototype virus could somehow infect the magical creatures in the Harry Potter universe. This would lead to a crisis where normal spells and magic can't stop the infected creatures. Harry and his friends would have to seek help from Mercer, who understands the virus. Together, they could try to find a way to either cure the infected or contain the spread. Mercer might use his own knowledge of the virus and his powers to fight the infected creatures, while Harry uses his magic skills to protect Mercer from magical attacks.
Hermione Granger and Alex Mercer would also have an interesting dynamic. Hermione is known for her intelligence and thirst for knowledge. She would be eager to study Mercer, his powers, and the science behind the Prototype virus. Mercer might be initially annoyed by her questions but would eventually come to respect her intelligence and might even learn a thing or two from her about the wizarding world.
Maybe there's a prophecy in both worlds that foretells of a combined force from the wizarding world and the ninja world being the only way to defeat an ultimate evil. So, Naruto and Harry are brought together. They face challenges in understanding each other's cultures and powers at first. But as they train together, they find ways to merge their skills. For instance, Naruto's rasengan could be enhanced with a magical spell to create a more powerful attack.
A possible plot is that a powerful magical artifact in the wizarding world is actually a key to an ancient Stargate on Earth that has been long forgotten. Harry and his friends, along with the Stargate team, embark on a quest to find this Stargate. Along the way, they face challenges from dark wizards who also want the artifact for their own evil purposes and the Goa'uld who sense the power of the artifact. They must work together, using both magic and technology, to reach the Stargate and unlock its secrets.
One possible plot could be that the One Ring somehow finds its way into the wizarding world. Harry and his friends discover it and realize its dark power. Meanwhile, some Hobbits from Middle - earth are accidentally transported to Hogwarts. They team up with the Hogwarts students to keep the Ring away from the dark wizards who want to use it for evil, just like Sauron in their world.
Well, for a Harry Potter and Naruto crossover fanfic, a cool plot could be that there is a dark force emerging that is a combination of Voldemort's magic and the Akatsuki's evil intentions. Harry Potter and Naruto then join forces. They start by training together, with Naruto teaching Harry some basic ninja moves like tree - walking to improve his physical abilities, and Harry teaching Naruto some basic spells for protection. Along the way, they gather their friends from both worlds, Hermione and Sasuke for example, to fight against this new threat.
A plot could involve Voldemort learning about the power of ADAM in Bioshock. He sees it as a way to gain ultimate power and sends his Death Eaters to Rapture to steal it. Meanwhile, a group of Aurors follow them, and then there's a three - way battle between the Aurors, Death Eaters, and the denizens of Rapture.
One idea could be that Voldemort discovers a way to enter the DC universe and teams up with some of the Justice League's villains like Lex Luthor. The Hogwarts students and the remaining members of the Justice League have to join forces to stop them. The wizards can use their magic while the superheroes use their powers.
One idea could be a spell gone wrong in the wizarding world that somehow transports some Hogwarts students to Forks. They then have to deal with the vampire and werewolf situation there while also trying to find a way back home. Maybe Hermione could be intrigued by the science - like aspects of the Twilight world that seem so different from magic.
One possible plot could be that the Death Eaters in the Harry Potter world team up with the Akatsuki from Naruto. Harry and his friends, along with Naruto and his comrades, have to join forces to stop this powerful alliance. They could learn from each other's magic and jutsu. For example, Naruto could teach the wizards about chakra control, and the wizards could show Naruto some advanced spells for defense and offense.
One idea could be that Quentin Coldwater from 'The Magicians' accidentally travels to the world of Harry Potter. He could be mistaken for a new exchange student at Hogwarts. He might try to use his Brakebills knowledge, like his unique spellcasting techniques, which would both amaze and confuse the Hogwarts students.