One idea could be that the One Ring somehow finds its way into the wizarding world. Maybe a Hogwarts student stumbles upon it during a field trip in an ancient forest. Another could be a wizard or witch from Harry Potter's world traveling to Middle - earth to study their different forms of magic. A third idea is that the characters from LOTR end up at Hogwarts to seek help in fighting Sauron, and they learn about magic wands and spells.
In a Harry Potter LOTR crossover fanfiction, it would be fascinating if the Fellowship of the Ring and the Trio from Harry Potter joined forces. They could combine their unique skills. The magic of Harry Potter's world like the Patronus charm could be used against the dark forces in Middle - earth. And the elves in Middle - earth could teach the wizards in Hogwarts about their ancient and powerful magic. For example, Legolas could show them how to be more in tune with nature in their magic. The wizards could then use this knowledge to improve their own spells and incantations.
Well, for a Harry Potter LOTR crossover fanfiction, a cool concept would be the Sorting Hat being used to sort the characters from LOTR. Aragorn could be a Gryffindor for his courage. Frodo might be a Hufflepuff as he is very loyal. And then, the Dementors could be used as a new threat in Middle - earth. The Ring - wraiths and Dementors could have some sort of interaction. Also, imagine Gandalf teaching at Hogwarts for a while and sharing his knowledge of magic with the young wizards and witches.
In a Harry Potter and LOTR crossover fanfiction, black magic could take on different forms. In the Harry Potter universe, it's often associated with the Dark Arts like the Unforgivable Curses. In LOTR, dark magic is related to Sauron and his minions. A crossover might explore how wizards from Harry Potter's world react to the more ancient and malevolent black magic of Middle - earth.
One idea could be that the One Ring somehow ends up in the world of Pirates of the Caribbean. Jack Sparrow, being the wily character he is, gets wind of its power and decides to either use it for his own gain or sell it to the highest bidder. The Fellowship, in turn, has to track it down in this strange new world full of pirates, sea monsters, and cursed treasures.
One idea could be to have the characters from Narnia and LOTR meet in a magical realm that exists between their two worlds. For example, Frodo and the hobbits could encounter Aslan. Another could be a story where the One Ring somehow ends up in Narnia and the Narnians have to deal with its corrupting influence, while Aragorn and his companions come to Narnia to help. Maybe the White Witch could team up with Sauron, creating a more powerful and menacing villain for the combined forces of Narnia and Middle - earth to face.
Here's an idea. Buffy and Xander end up in Lothlórien. The elves are initially suspicious but then realize they are there to help. Buffy could use her agility and strength to assist in defending the forest. In another scenario, the Nazgûl could cross over to Buffy's Sunnydale. This would be a huge threat, and Buffy would have to use her knowledge of fighting demons along with the help of her friends and the weapons of Middle - earth (if they can get them) to defeat these powerful enemies.
One more theme is the exploration of different cultures. The wizarding world in Harry Potter has its own unique culture, with things like Quidditch and the Houses. Middle - earth in LOTR has different races like hobbits, elves, and dwarves. In the crossovers, characters get to experience and learn about these different cultures. For instance, Hermione might be fascinated by the elvish language and lore, while Legolas could be intrigued by the spells at Hogwarts.
Another plot could involve the Rings of Power in LOTR having some sort of connection to the magic in Narnia. For example, the White Witch might be interested in obtaining the One Ring to increase her power. This could lead to a conflict where the forces of good from both Narnia and Middle - earth have to join together to stop her. The different races in both worlds, such as the hobbits, dwarves, elves in LOTR and the talking animals in Narnia, could interact in unique ways during this conflict.
One idea could be Sesshomaru arriving in Middle - earth in search of a powerful relic. He might cross paths with Aragorn and initially be seen as a threat. Another could be that he forms an alliance with the elves, as they both have an air of elegance and long - lived existence. Maybe he gets involved in the battle against Sauron, using his demonic powers in a way that surprises both friend and foe.
One idea could be to have nanomachines in the Harry Potter universe that are secretly created by a dark wizard. They could be used to control the minds of students, but the protagonists discover this and use magic inspired by LotR spells to combat it. For example, a Patronus charm with a LotR - like incantation to repel the nanomachine - influenced dark magic.
A really interesting way would be if there was a prophecy in both worlds about an immortal being who would bring balance. A character from Harry Potter, like Dumbledore who has some form of extended life through his powerful magic, and a character from LOTR, say Gandalf who is a sort of immortal spirit, could be the key players. They could discover that they are both part of this prophecy and have to work together. Their combined wisdom and power would be used to fight against the common threats in both worlds, such as the dark wizards in Harry Potter and the forces of Sauron in LOTR. This would involve a lot of world - building and character development as they bridge the gap between their two very different universes.
In a crossover fanfiction, black magic might be the catalyst for the meeting of the two worlds. Maybe a dark sorcerer in Middle - earth uses black magic to open a portal to the wizarding world of Harry Potter. This would bring about chaos as the characters from both sides have to deal with this new and unfamiliar form of black magic. The wizards from Hogwarts may be shocked at the more primal and ancient black magic they encounter in Middle - earth.