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.
You can start by looking on fanfiction.net. It has a large collection of various fanfictions, and you're likely to find some Buffy LotR crossovers there. Another place is Archive of Our Own (AO3). It has a well - organized tagging system, so you can search specifically for Buffy and LotR crossovers.
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.
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.
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.
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.
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 great Buffy and Charmed crossover fanfiction is 'The Power of Two Worlds'. It combines the unique magic systems of both shows in an interesting way. The characters from Buffy and Charmed have to team up to fight a common enemy that threatens both their universes. The author does a good job of keeping the personalities of the original characters intact.
I really liked 'When Hunters Meet Slayers'. It's a well - written fanfic that does a good job of melding the two universes. It has some great action scenes where Sam uses his research skills to help Buffy and her gang figure out a new type of demon that has emerged, which is a combination of a demon from 'Supernatural' and one from 'Buffy'.
There's also 'Bones Meets Buffy'. In this fanfic, the two universes collide when a series of murders in Buffy's Sunnydale seem to have a connection to some archaeological finds that Bones is researching. It's a fast - paced story that keeps you on your toes. The author does a good job of maintaining the personalities of both sets of characters. For example, Bones is as logical and matter - of - fact as ever, while Buffy is her brave and sassy self.