One great one could be where the reader is a hobbit and joins Frodo's journey. It gives a really unique perspective as a small, underestimated character in a grand adventure.
One interesting 'Lord of the Rings fanfiction' story could be about an alternate ending where Frodo fails to destroy the One Ring at Mount Doom. Another might be a story that focuses on the adventures of Legolas and Gimli after the main events of the trilogy. There are also fanfictions that explore the backstories of minor characters like Tom Bombadil in more detail.
One interesting story could be about Naruto finding the One Ring and having to resist its power with his ninja will. Maybe he teams up with Frodo and they teach each other about their respective worlds.
Well, in some 'OC x The Lord of the Rings fanfiction', the OC could be an elf from a different part of Middle - earth that has a different culture and set of abilities. This OC might meet Legolas and they form a bond while sharing their respective elven knowledge. Also, there are fanfictions where the OC is a dwarf with a special talent for forging unique weapons that could potentially change the course of the war. And of course, there are stories where the OC is a human with a mysterious past that is somehow linked to the One Ring.
Perhaps some of the Hollows from Bleach escape into Middle - earth. The Fellowship of the Ring then has to team up with Soul Reapers who follow the Hollows there. The Soul Reapers could teach the Fellowship about the spiritual nature of the Hollows, and together they work out strategies to defeat them. For example, Aragorn could learn some combat techniques from the Soul Reapers, and in return, the Soul Reapers could gain knowledge about Middle - earth's geography and lore from Aragorn.
In these fanfictions, a frequent theme is the breaking of gender norms. Middle - earth has its own set of traditional gender roles, and the crossdressing reader character challenges those. Also, there's often a theme of adventure. The crossdressing doesn't just stop at a change of clothes, but it can lead the character into new adventures, like infiltrating an enemy camp disguised in a different gender's attire, adding an extra layer of intrigue to the story.
Well, perhaps the prophet oc is from a different realm and is brought to Middle - earth. In the fanfiction, they could have a special connection with the One Ring. Instead of being corrupted by it, they use their prophetic powers to understand its true nature and find a way to destroy it without having to go through the long and arduous journey that Frodo had to endure. They could be like a guiding force for the Fellowship, using their knowledge of future events to keep them safe.
Well, in this kind of fanfiction, the reader - insert might train some of the characters in MMA. Imagine teaching Aragorn some new combat moves. He could then incorporate these into his own fighting style and be even more formidable in battles. Also, the reader could participate in the great battles like Helm's Deep, using their MMA knowledge to hold off the enemy waves.
Well, in 'lord of the rings si fanfiction', there might be concepts such as new races emerging in Middle - earth. Maybe a race that was hidden before decides to show itself during the War of the Ring. Also, different interpretations of magic are interesting. Some fanfictions might explore how magic could be used in ways not shown in the original books, like using magic to heal not just physical wounds but also mental traumas.
One interesting aspect is how their simple lives in the Shire are often expanded upon. In fanfiction, Hobbits may have more complex relationships with the other races. Their love for food and comfort can be explored in different ways, like how they might use their food knowledge to help others on a journey. Also, their courage, which was shown in the original works, can be further developed in fanfiction, perhaps in situations where they have to stand up to greater threats on their own.
A crossover with the Star Wars universe would also be cool. The Jedi could be like the wizards in Middle - earth. Maybe Aragorn could learn some lightsaber skills from the Jedi and use them against the orcs. And the Force could have an interesting interaction with the magic in Middle - earth.