A possible plot is that Sesshomaru is lured to Middle - earth by the power of the One Ring. However, instead of being corrupted by it, he decides to destroy it, seeing it as a dangerous object that could disrupt the balance not just in Middle - earth but also in his own world. Along the way, he teams up with Gandalf, and together they face many challenges as they journey towards Mordor.
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 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.
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.
A great plot could be a crossover where Sesshomaru and Rin end up in a magical realm that is a mix of their two worlds. Here, they meet new characters who mistake them for powerful beings from their own lore. They have to work together to find a way back home while also dealing with the power struggles in this new realm. Or perhaps a plot where Rin gains some of Sesshomaru's powers accidentally and Sesshomaru has to train her to control them, which leads to a deeper bond between them as they face challenges from other demons who are envious of Rin's newfound powers.
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.
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.
Sesshomaru's speed and agility would be a great asset in Middle - earth. He could easily outrun orcs and other foes. His sword, Tenseiga, which has the power to resurrect, could also have an interesting impact. For example, if he were to use it on a fallen hero, it could change the course of a battle.
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.
One idea could be that the dragons from Wings of Fire find themselves in Middle - earth. They might get involved in the War of the Ring, using their unique dragon abilities to turn the tide of battles. For example, their fire - breathing could be a great asset against the orcs.
Perhaps Kikyo is in trouble and Sesshomaru is the only one who can save her. But in the process, he discovers a side of her that he didn't expect, like her strong will and determination even in the face of death. This could lead to a newfound respect between them and the start of an unexpected friendship or more.