One interesting idea could be exploring the backstories of minor characters in more depth. For example, the soldiers in Hellsing who often get overshadowed by the main characters. Their experiences and how they deal with the horror around them can add a new dimension to the fanfiction.
Another idea might be to create alternate timelines. Maybe in one timeline, Alucard makes different choices that lead to a completely different power dynamic within Hellsing. This can lead to exciting new storylines and character developments.
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.
Alucard is often very popular. His mysterious and powerful nature makes him a fascinating character to write about. Writers can explore his past, his powers, and his complex relationship with Hellsing.
Another idea is to do a crossover where Percy gets transported to the Hellsing universe. He'd be completely out of his element and have to adapt to the darker, more violent world. He might initially be seen as a threat by the Hellsing organization but then prove his worth when they realize his powers can be useful in their fight against the undead. Maybe he could even teach them some new combat techniques inspired by his demigod training.
One idea could be an OC who is a half - vampire, half - human hybrid trying to find their place in the Hellsing world. They could have unique powers that are a combination of both vampire and human traits, and struggle with their identity throughout the story.
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.
Another possibility is a crossover with 'Tokyo Ghoul'. The ghouls' unique abilities and the vampire - hunter theme of 'Hellsing' could make for a really intense and dark fanfiction. The struggle between the two species in a shared world would be full of drama and action.
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.
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.
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.