A cool plot for this crossover fanfiction could be that the Volturi, thinking they can control the power of Middle - earth, send some of their members to retrieve the One Ring. But once there, they realize that the magic and power dynamics are far more complex than they thought. Meanwhile, Frodo and his companions encounter a strange group of 'undead - like' beings (the vampires) and are initially suspicious. As the story progresses, they might find common ground in their fight against the forces of darkness. The different cultures and powers in both universes would create a lot of opportunities for exciting interactions and plot twists.
One idea could be that the vampires from Twilight are drawn to the power of the One Ring. They might see it as a source of great power that could enhance their abilities. Another plot could involve a character from Twilight getting transported to Middle - earth and having to adapt to the new world while also dealing with the threat of Sauron. Or perhaps the werewolves from Twilight could form an alliance with the Ents in Middle - earth, as they both have a connection to nature.
Well, a possible plot could be that Bella, being a bit of an outcast in her own world, finds herself in Middle - earth. She could be mistaken for an elf at first due to her pale beauty. And then she gets caught up in the struggle against Sauron. Maybe she uses her 'humanity' in a unique way to inspire the people of Middle - earth. Also, the Cullens could be seen as a new kind of 'race' in Middle - earth, and their presence could disrupt the balance between good and evil. The Cullens' ability to resist the allure of the One Ring could be an interesting aspect to explore as well.