One common theme is loyalty. Pip is always loyal to Seras, and this is often explored in the fanfics. For example, in many stories, he will go to great lengths to protect her.
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.
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.
There's also 'Seras and Pip: A New Chapter'. This fanfic focuses on their adventures together in a post - apocalyptic world where they have to fight against new threats while also dealing with their growing feelings for each other. It's quite action - packed.
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.
In Hellsing Ultimate Abridged Fanfiction, it's a fan - created work that builds on the Hellsing Ultimate world. It could involve the same characters like Alucard, Seras Victoria, and Integra Hellsing. The abridged aspect means it may cut out some parts of the original story and add its own twists. Maybe it gives a different perspective on the battles between the vampires and the Hellsing organization. It could also introduce new dialogues that are full of inside jokes and references that fans of the original would appreciate.
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.