One idea could be a crossover where Naruto is somehow transported to the Hellsing universe and meets Seras. Maybe Naruto's unique powers, like his shadow clones, could be used in battles against the vampires in the Hellsing world. Seras, with her vampire abilities, might initially be seen as an enemy to Naruto but they could end up teaming up.
Naruto and Seras might start off being cautious around each other. Naruto, being from a world of ninjas, may not trust a vampire like Seras right away. But as they get to know each other, they could become friends. Seras could be fascinated by Naruto's positive attitude and his never - give - up spirit.
One great Devil x Seras Hellsing fanfic could be 'The Unlikely Bond'. It delves into how their relationship might develop in a world full of supernatural chaos. The story might explore how Devil's nature and Seras' vampiric state interact in unique ways.
Overall, Hellsing Ultimate is reasonably close to the manga. Some minor details might be different, but the main story and the core characters remain consistent. The animation style also does justice to the manga's dark and intense atmosphere.
One interesting idea could be their unique relationship dynamic. Seras is a vampire and Pip is a human, so exploring how they bridge the gap between their two worlds can be really engaging.
First, study the characters of Devil and Seras from Hellsing. Know their personalities, backstories, and how they interact in the original work. Then, come up with a unique plot. For example, you could set it during a new vampire uprising in the Hellsing world where Devil and Seras have to rely on each other more than ever.
You could also search for fanfictions that focus on the character development of both Seras and Pip within their relationship. Fanfictions that explore their backstories and how those influence their relationship in the 'hellsing' universe are often well - written and interesting.