In a crossover, Merida could teach the Hogwarts students about her own form of magic, like the spells she uses for her archery and how she communicates with the animals in her world. At the same time, she could learn about the complex spells and potions of the wizarding world. This exchange of knowledge could lead to some exciting adventures, like using a combination of both types of magic to defeat a common enemy.
Perhaps in this fanfiction, the magic in the Harry Potter world starts to fade because of the influence of the 'Brave New World' ideology. Wizards are made to believe that their magic is a form of chaos that needs to be controlled. Harry Potter, being the hero, goes on a quest to find the source of this change and discovers that Voldemort is somehow involved with the people from 'A Brave New World'. He then has to battle not only Voldemort but also the new order that is trying to reshape the wizarding world.
A crossover with How to Train Your Dragon would be cool. Merida could meet Hiccup and they could compare their skills in taming wild creatures. Merida with her horse and archery skills, and Hiccup with his dragon - riding and training. They could team up to fight a common enemy, perhaps a dragon that has gone rogue and is threatening both their lands.
One idea could be that the Grimm creatures start to appear in the wizarding world. Harry and his friends have to use both magic and knowledge of Grimm lore to fight them. For example, they might discover that some dark magic has opened a portal between the two universes.
One idea could be that the Order of the Phoenix teams up with the Kingsman. The wizards could use some of the Kingsman's high - tech gadgets and fighting skills, while the Kingsman could learn about magic. For example, Harry Potter could be trained by a Kingsman agent in hand - to - hand combat, which would give him an edge in battles against Death Eaters. Another plot could involve a villain who is a hybrid of a dark wizard and a criminal mastermind that the Kingsman usually deal with. The two groups would have to combine their resources to stop this new threat.
Maybe some of the Hogwarts students are sent to the Naruto universe. They would be amazed by the ninja techniques. Hermione, for instance, would be eager to study how chakra works and might even find a way to combine it with magic. Harry could learn some ninja stealth skills which would be useful in his fight against Voldemort.
One idea could be that the Hogwarts students discover Halo technology. Maybe Harry and his friends find a crashed UNSC ship and start to study its advanced weaponry and shields. They could try to adapt the technology to defend Hogwarts from Voldemort. Another plot could involve Master Chief being sent to the wizarding world by accident. He then has to navigate the strange new world of magic while also dealing with threats that his usual Halo combat skills might not be fully equipped for.
One idea could be that the Hogwarts students find a crashed Firefly - type spaceship in the Forbidden Forest. The crew of the ship could be injured and the students use magic to heal them. Then they exchange knowledge, with the Firefly crew teaching about space travel and the Hogwarts students showing off their magical skills.
Well, a cool plot could be that the Death Eaters are interested in the power of alchemy. They try to capture Edward to force him to use alchemy for their evil purposes. Harry, Ron, and Hermione then rescue him. In return, Edward offers to help them with his alchemical skills in their fight against Voldemort. For example, he could use alchemy to create unique magical items or barriers. Also, Alphonse could befriend some of the Hogwarts ghosts and learn about the secrets of the castle from them, which could be useful in the fight against the dark side.
One idea could be that Ranma somehow gets transported to the wizarding world. Maybe he's mistaken for a new kind of magical being. He could use his martial arts skills in the Triwizard Tournament, which would be a unique take on the challenges. Another plot could involve Harry Potter and his friends visiting Japan and stumbling upon Ranma's crazy world of cursed springs and martial arts battles. They could learn some of Ranma's techniques to use against Voldemort.
Perhaps the Voldemort threat in the Harry Potter world is somehow connected to a Darkness - like entity in Destiny. Harry and his friends must learn new combat techniques from the Guardians or other Destiny characters to combat this new, more powerful form of the Dark Lord. They could also exchange magical and technological knowledge, which would be really cool to see.