A great idea for a Hellsing fanfic is to explore the world outside of the Hellsing Organization's main base. What kind of vampires or other supernatural creatures lurk in the far - flung corners of the world? You could also write a fanfic centered around Integra's training as a young girl, how she overcame the challenges and expectations placed on her as the heir of the Hellsing family. This could include her first encounters with the supernatural and how she learned to handle them, which would give a deeper understanding of her character.
An interesting fanfic concept could be centered around Alucard facing a new type of supernatural threat that he's never encountered before. This could be a group of demons with powers that are different from the vampires he usually battles. The fanfic could detail his struggle to understand their abilities and find a way to defeat them. It could also show how the Hellsing organization reacts to this new threat, whether they trust Alucard completely to handle it or if they try to interfere in his methods.
One idea could be the reader being a new recruit in the Hellsing organization. Alucard becomes intrigued by the reader's unique courage and tenacity. He starts to protect the reader during missions, leading to a growing bond between them.
One idea could be a crossover where Alucard from Hellsing meets the vampires in Twilight. Maybe they have a battle over territory or different views on being a vampire. Another could be a story where a character from Twilight gets involved with the Hellsing Organization and has to adapt to their more violent and no - nonsense way of dealing with the supernatural.
Another idea is a story where some of the Naruto villains team up with the antagonists from Hellsing. Orochimaru might be interested in the vampire technology or the powers of the vampires in Hellsing. He could try to use it to further his own goals, and this could lead to a complex and action - packed plot with both Naruto and Hellsing characters caught in the middle.
One idea could be creating an OC who is a new recruit in the Hellsing Organization. They could have a unique power, like the ability to control shadows, which adds an interesting dynamic to the battles against vampires. Another idea is an OC that is a vampire with a conscience, struggling between their blood - thirsty nature and the human values they still hold. This can create complex moral dilemmas within the Hellsing universe.
You could also focus on an alternate universe where the events of Hellsing Ultimate took a different turn. Maybe Integra made different decisions early on, which led to a completely different power structure within the organization and different battles against the forces of evil.
The unique interpretations of the characters. For example, seeing a different side of Alucard or Seras Victoria. It could be a softer side, or a more complex backstory added to them.
One idea could be to have Alucard from Hellsing meet the vampires in Twilight. Maybe Alucard would be amused by their 'tame' nature compared to his own blood - thirsty self. He could show them what a real vampire is capable of in a fight.
One idea could be a crossover with 'Berserk'. The dark and gritty worlds of both could blend well. The vampires from Hellsing could interact with the Apostles from Berserk, leading to epic battles. Another could be a crossover with 'Fullmetal Alchemist'. Imagine the alchemists trying to understand the nature of the vampires in Hellsing. Maybe they'd try to find a way to use alchemy on them.
An interesting OC for Hellsing Ultimate fanfiction could be a young orphan who is taken in by the Hellsing Organization and trained to be a part of their ranks. This OC could have a special connection to one of the main characters, like Alucard. They might start off as a naive and inexperienced recruit but grow into a strong and reliable member of the team, facing various challenges and moral dilemmas along the way.