One idea could be that the OC is a healer who joins Aragorn's journey. They meet during a battle where Aragorn is injured and the OC uses their unique healing skills to save him. This starts a bond between them as they travel together, with the OC providing medical aid to the whole fellowship.
Maybe the OC is a mysterious elf-like being from a far - off land. Aragorn is initially suspicious but as they face challenges together, like fighting off orcs in a forest, he realizes the OC has valuable skills in stealth and archery. Their relationship grows as they teach each other their different ways of survival.
The OC can be integrated through a common goal. For example, if the OC is a ranger from a different region but shares Aragorn's goal of protecting Middle - earth from Sauron. They can meet during a scouting mission and decide to combine their forces.
Your OC could be a ranger who has a vendetta against a powerful orc chieftain. The orc has been raiding villages and killing innocents. The OC sets out on a mission to hunt down the orc. Along the way, they get involved in a larger conflict between the forces of good and evil. They meet other characters from the Lord of the Rings world, like Aragorn or Legolas. This plot allows for a lot of action and character development as the OC battles their personal enemy while also contributing to the greater fight against Sauron.
Maybe the female elf OC is a healer. She comes across a wounded character from the Lord of the Rings world. As she nurses him back to health, they start to bond. They then journey together to find a rare herb that can cure a deadly disease spreading in Middle - earth. Along the way, they face orcs and other perils, and their relationship deepens.
One idea could be creating an original character who is a lost heir to one of the kingdoms in Middle - earth. This character could have been hidden away during a great war and now emerges to reclaim their rightful place. Another idea is an OC who is a skilled ranger from a previously unheard - of tribe in the forests. They could cross paths with the Fellowship and offer their unique skills to the cause.
I'm not sure if there are many Vampire Aragorn fanfictions out there. However, you could start by checking some popular fanfiction websites like Archive of Our Own. They have a large collection of Lord of the Rings fanfictions. You might need to use the search function and type in 'Vampire Aragorn' specifically to find relevant works.
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.
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.
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.
Another theme might be isolation. As a vampire in a world that mostly consists of non - vampires, Aragorn would probably face a great deal of ostracism. This could lead to a sense of loneliness and a search for his place in the world, much like his character's journey in the original Lord of the Rings but with a very different set of circumstances.
One idea could be exploring the backstory of a minor character in the game. For example, a Uruk-hai who has a change of heart and decides to fight against Sauron's forces instead. It would add an interesting twist to the existing lore.