One possible plot could be that Eragon somehow gets transported to Middle - earth. He could meet Aragorn and learn from his leadership skills. Maybe they would join forces to fight a common enemy like a new breed of dark creatures created by a combination of Sauron's magic and the dark forces from Eragon's world.
Saphira, Eragon's dragon, and Gwaihir, the great eagle in Lord of the Rings. These two powerful flying creatures could form an alliance. They could communicate in their own ways and work together to provide aerial support during battles, as well as help in transporting important characters across great distances in Middle - earth and Eragon's world.
A really cool 'Lord of the Rings' crossover fanfiction is when it crosses with the DC Universe. Superman or Wonder Woman in Middle - earth would be quite a sight. They could use their powers to fight the forces of Sauron. This kind of crossover often explores how the extraordinary powers from the DC side would impact the more medieval - style world of 'Lord of the Rings'.
One idea could be a vampire character who is drawn to the power of the One Ring. Maybe this vampire, being a creature of the night and often associated with darkness and power -hunger, sees the ring as a means to gain ultimate power in Middle - earth. For characters, you could have a half - vampire, half - elf who struggles with their dual nature while being caught in the events related to the ring.
To write a good vampire Lord of the Rings crossover fanfiction, you need to focus on character development. If you're introducing a vampire lord, make him a complex character. Give him motives that are more than just 'I want to rule the world'. Maybe he has a tragic past that led him to his current state. Also, make sure the language you use fits the tone of both worlds. The Lord of the Rings has a very specific, almost archaic style of speaking in places, and your vampire characters should also have a voice that is consistent with their nature. And don't forget about the pacing. You don't want to rush the story, especially when it comes to the big moments like battles or revelations about the ring.
One way it could work is by having Naruto characters in Middle - earth. For example, Naruto with his shadow clone jutsu could be a great asset in the battles against Sauron's forces. He could create clones to confuse the orcs and trolls. Meanwhile, characters from 'Lord of the Rings' could learn from Naruto's ninja skills. Frodo might pick up some stealth techniques from Naruto's training.
One common theme is the clash of different cultures. Buffy comes from a modern world with a certain set of values and fighting styles, while Middle - earth has its own ancient and more traditional ways. So, the stories often explore how these differences play out when she arrives there.
A possible crossover would involve Merlin's struggle with the dark forces in Middle - earth. He is used to fighting against Morgana and her minions in Camelot, but in Middle - earth, the enemies are Sauron and his armies. His skills in magic and strategy would be put to the test. He could also form an alliance with Aragorn, sharing his knowledge of leadership and magic to help Aragorn in his quest to become king.
You can start by looking on fanfiction websites like Archive of Our Own. They have a large collection of fanfictions, and you can search specifically for 'Lord of the Rings' crossovers. Use relevant tags to narrow down your search.
Merlin and Frodo could have an interesting interaction. Frodo, burdened with the One Ring, might seek Merlin's advice on how to resist its power. Merlin, having dealt with dark magic before, could offer some insights.