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 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.
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.
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 that Buffy and the Scooby Gang find themselves in Forks and have to deal with the vampire situation there. They might initially clash with the Cullens due to different ways of handling vampires. Another could be a love triangle between Buffy, Edward, and Angel. It would add a new layer of complexity to the relationships already present in both series.
One idea could be that Buffy and the Highlander team up to fight a common supernatural enemy that is causing chaos in both their worlds. Maybe a powerful demon that can't be killed easily, and the Highlander's immortality and Buffy's slayer skills are needed to defeat it.
Another concept could be that a powerful demon from Buffy's world is attracted to the unique vampires in Forks. This demon starts causing chaos, and Buffy and the Cullens have to team up to stop it. During this time, they learn from each other's fighting styles and supernatural knowledge. For example, the Cullens could teach Buffy more about their kind of vampire lore, while Buffy could show them some of her combat techniques against demons.
One plot idea could be that the One Ring somehow finds its way into the X - Men universe. Maybe Magneto senses its power and wants to use it for his own purposes, believing it could give him ultimate control over mutants and humans alike. The X - Men, on the other hand, realize the danger of the Ring and try to stop Magneto from getting his hands on it. Another idea is that some of the LOTR characters are reincarnated as mutants. For example, Aragorn could be a mutant with enhanced leadership abilities and physical prowess, and he teams up with the X - Men to fight a common enemy that threatens both Middle - earth and the mutant world.
A possible plot could involve a powerful and ancient evil that has ties to both the Buffyverse and the world of Sons of Anarchy. This would force the two groups to work together. The Sons of Anarchy members could provide muscle and their knowledge of the criminal underworld, while Buffy and her friends bring their supernatural expertise. It would be a clash of two very different worlds and cultures, but also a chance for some great character development and action.