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.
Another interesting concept is that some of Naruto's enemies in his own world start to use dark magic similar to that in the Hellsing universe. Integra sends some of her agents to Naruto's world to investigate. Naruto and Integra meet during this investigation and find that they can share knowledge and resources to combat this new threat.
Perhaps a story where Naruto is somehow transported to the Hellsing world. He could learn about their way of dealing with vampires and other dark creatures. Integra might initially be suspicious of him but come to respect his strength and determination. They could then work together to face a common enemy, like a powerful vampire lord that threatens both their worlds.
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.
A great 'hellsing integra vampire fanfiction' could be a story where Integra has to face a vampire uprising without Alucard by her side. She has to rely on her own wits and the other members of the Hellsing organization. This would show her growth as a character and how she can handle difficult situations independently. It could also introduce new characters within the Hellsing organization that play important roles in the battle against the vampires.
One idea could be Integra being transported to the Naruto world and having to adapt to its ninja ways. Maybe she uses her unique skills from her own world to make a mark in the Naruto universe.
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.
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.
A common theme is the power dynamic between them. Alucard is extremely powerful, but Integra is his master. Fanfictions often play with this dynamic, showing how they navigate it. Another theme is loyalty. Alucard's loyalty to Integra is a key element, and it's explored in different scenarios in fanfiction.
He'd bring a new sense of hope. The Hellsing organization is often mired in dark and violent affairs. Naruto's positive attitude could be a breath of fresh air for the members. For example, Seras might be inspired by Naruto's determination to protect those he cares about.
A good Hellsing Integra x Alucard fanfiction I've read is 'Blood and Roses'. It's unique as it not only focuses on their relationship but also on the lore of the Hellsing world. It shows Integra's growth as a character while maintaining her connection with Alucard. Additionally, 'Twilight Alliance' is worth checking out. It presents a different side of their relationship, where they face challenges that test their loyalty and feelings for each other.
One interesting plot idea could be a story where Seras and Integra have to team up against a new and powerful vampire threat. They start to understand each other on a deeper level during their battles and form a stronger bond.
Well, Naruto's skills and personality would clash with the dark and gritty world of Hellsing. Naruto's optimism might start to change the gloomy atmosphere in Hellsing. He could also introduce some new jutsu and combat styles, which would be really interesting to see how the Hellsing crew reacts to it.