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.
In terms of abilities, Sesshomaru's regenerative powers would be quite useful. If he were injured in a battle against the forces of Sauron, he could quickly heal and get back into the fight. His Bakusaiga, which can cut through dimensions, could potentially be used to access hidden or protected areas in Middle - earth, like the paths to the Undying Lands or the secret lairs of evil creatures.
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.
The telepathic abilities of some Star Trek species, like the Betazoids, could blend interestingly with the mind - related magic in Lotr. For example, a Betazoid could potentially communicate with wizards or other magically - attuned characters in Middle - earth on a different level. Also, the transporter technology in Star Trek could have a strange interaction with the magical portals or teleportation - like spells in Lotr.
If we consider how the Percy Jackson characters would fit into LOTR's world, we also have to think about the different races in Middle - earth. The demigods might have different relationships with the hobbits, dwarves, elves, and men. The hobbits could be both curious and a bit scared of the demigods at first. The dwarves might be interested in the demigods' weapons and their fighting abilities. The elves, with their own magic and long - lives, could compare and contrast their existence with that of the demigods. And the men of Middle - earth would likely be both cautious and eager to use the demigods' powers for their own purposes in the fight against Sauron.
Definitely Legolas and Will Turner. Legolas with his amazing archery skills could teach Will some new techniques, and Will could show Legolas the ropes of being a pirate. They both have a sense of honor and loyalty, so they would make an interesting duo.
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.
Sesshomaru would likely be aloof and haughty at first. He might look down on Naruto as a mere human. Naruto, being his friendly self, would probably be curious about Sesshomaru and try to start a conversation. Sesshomaru might respond with cold remarks, but Naruto's persistence could gradually break through his icy exterior.
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.
A lesser - known but excellent piece is 'Ringbearer Buffy'. Here, Buffy is somehow made into a ringbearer. It's a unique take on both franchises. The story delves deep into how Buffy's personality and her experiences as a slayer would affect her role in the quest to destroy the One Ring. It has some great character development and some really intense action scenes.
One way to blend the themes is through the idea of sacrifice. In LOTR, many characters sacrifice for the greater good, like Boromir. In Narnia, Aslan sacrifices himself. A fanfiction could explore how characters from both worlds understand and are affected by sacrifice. Also, the theme of the power of nature is present in both. The ents in LOTR and the talking animals and plants in Narnia can be part of a story that shows how nature can be a force for good against the encroaching evil in both worlds.
Nanomachines could be introduced as a foreign element from a different dimension. In the fanfic, they might be accidentally brought into the magical worlds of LotR and Harry Potter. For instance, a wizard in the Harry Potter world experiments with a new spell that creates a portal, and through this portal, nanomachines from a high - tech future world enter. In LotR, they could start affecting the ents, making them act in strange ways. The characters would then have to figure out how to deal with this new threat.