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 plot idea could be Naruto being transported to the Hellsing universe and having to deal with the vampires there using his ninja skills. He might form an alliance with the Hellsing Organization and help them fight against a new, powerful vampire threat.
One idea could be a crossover where Naruto's world is invaded by supernatural forces from the Hellsing universe and Integra has to team up with Naruto to fight them. Naruto's ninja skills combined with Integra's leadership and the power of the Hellsing organization would make for an exciting story.
A different plot could be that a curse has been placed on both Naruto and Seras. They start having visions of each other's past traumas. As they try to figure out how to break the curse, they discover that it was placed by an old enemy of both the ninja clans and the Hellsing organization. To break it, they need to find a magical artifact hidden in a dangerous dimension that is guarded by powerful demons and rogue ninjas.
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.
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.
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 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.
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.