The plot could involve the vampire OC using Alucard's power to fight against other supernatural beings. Maybe there's a power struggle in the vampire world and the OC has to prove themselves.
The plot might involve the vampire OC discovering the power of Rai gradually. At first, they don't understand it, but as they explore, they learn how to harness it. They could use it to protect their human friends or to gain a higher status among vampires. For example, they might use it to defeat a powerful vampire elder who is causing trouble in their territory.
One important aspect of Hellsing Alucard x OC fanfiction is the world - building. The writer needs to make sure that the OC doesn't disrupt the established lore of Hellsing too much. Alucard's character is complex, and any relationship with an OC should enhance that complexity. The fanfiction can also explore Alucard's past through the eyes of the OC, perhaps revealing new aspects of his long and bloody history that were not fully explored in the original series.
If there are humans in the story, they would probably be both afraid and fascinated by the vampire OC. The OC might have to hide their power at first to not scare the humans away. But as they get closer, the humans could become allies and help the OC in different ways. For example, a human hacker could help the OC find information about the origin of the Rai power. Also, other supernatural characters like werewolves might see the vampire OC as a threat and constantly be in conflict with them, while some might want to form an alliance for mutual benefit.
Some Alucard OC fanfiction focuses on an OC who is a reincarnation of someone from Alucard's past. This creates a sense of deja vu and a strong emotional connection between them. In one story, the OC is a bard who uses music to soothe Alucard's tortured soul. The music has a strange effect on him, and they develop a deep bond. There are also fanfictions where an OC is a vampire slayer with a special weapon that can actually harm Alucard. But instead of killing him, they end up in a situation where they have to work together against a horde of rogue vampires.
A prevalent theme in 'alucard x oc fanfiction' is the struggle between good and evil within Alucard. The OC usually plays a role in this internal battle. Additionally, themes of loyalty and betrayal can be found. The OC may have to prove their loyalty to Alucard, and there could be elements of betrayal from other characters in the story. Also, the exploration of immortality is common, as Alucard is immortal and the OC might be mortal, leading to complex emotional and plot developments.
In many vampire OC fanfictions, Harry can be a vampire. Fanfiction writers often like to re - imagine characters in different supernatural forms to explore new storylines. They might give Harry the typical vampire traits such as immortality, super strength, and a thirst for blood. This can lead to all sorts of interesting plot twists, like how he interacts with other characters who are either afraid of him or attracted to his new vampiric nature.
Identity is a big theme as well. The OC may have their own set of values and beliefs that are tested in the Hellsing world. They might question their own identity as they interact more closely with Alucard and the other characters. Also, themes of loyalty and betrayal can play out, especially when the OC has to choose between different factions or loyalties within the Hellsing universe.
Their relationship would change significantly. At first, Seras might be resentful towards Alucard for taking away her human life. But as she comes to terms with her new nature, she may grow closer to him. Alucard, who is used to being alone, now has someone he has a special connection with. They might start to rely on each other more, sharing secrets about the vampire world that only they can understand.