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.
One possible interesting plot could be an Assassin in Middle - earth. The Assassin could use their skills to infiltrate Mordor and take out key Sauron's lieutenants. They might team up with the Fellowship, sharing their stealth and combat techniques to help in the fight against the Dark Lord. For example, the Assassin could use their parkour skills to navigate the treacherous mountains around Mordor, getting to places that are hard for others to reach.
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.
Well, perhaps there's a villain in the Bleach universe who somehow obtains a Miraculous and starts using it for evil purposes. Ichigo and Ladybug have to figure out how to stop this new, powerful foe. Ladybug could teach Ichigo some of her strategic thinking, while Ichigo could show her how to better handle intense combat situations.
Perhaps in this crossover fanfiction, there is a prophecy in both the Naruto and Bleach worlds about a combined force that can save the multiverse from a great evil. Naruto and Yoruichi are the key components of this prophecy. At first, they don't get along as they have very different personalities and ways of doing things. Naruto is more straightforward and brash, while Yoruichi is sly and mysterious. But as they face challenges together, they learn to work as a team and combine their powers of ninjutsu and shinigami abilities to defeat the ultimate threat.
Another great crossover could be between Gandalf and Urahara. Gandalf is a wise and powerful wizard, and Urahara is a very intelligent and somewhat mysterious character in Bleach. They could share their knowledge of magic and strategy. Gandalf might be intrigued by Urahara's unique inventions and scientific approach to spiritual matters, while Urahara could learn from Gandalf's vast experience and wisdom in dealing with different races and powers.
A fanfic could have Shirou befriending the hobbits. He could teach them some self - defense skills using his martial arts knowledge. And in return, the hobbits could introduce him to the simple joys of Shire life. Maybe he even helps them in their quest to destroy the One Ring in some way.
A possible plot could be that a hobbit, who has a very charming personality, accidentally gathers a harem of female characters from different races. They join him on his journey, which is not just about destroying the One Ring but also about promoting unity among the different races in Middle - earth. This harem then becomes a symbol of hope and cooperation, showing that different races can coexist and work together.
Perhaps the gamer could start a new fellowship, not to destroy the One Ring but to study and preserve the magic within all the rings. They could travel across Middle - earth, gathering knowledge and artifacts related to the rings, facing challenges from both Sauron's forces who think they are trying to undermine their plans, and from those who fear the power of the rings being misused.
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.
A really interesting crossover is when the One Ring ends up at Hogwarts. It creates a whole new set of challenges as the wizards and witches have to figure out what this powerful and evil object is. Some of the professors try to study it while others worry about its corrupting influence on the students.