One possible plot could be that Naruto and his team are sent to the Hellsing universe on a mission. They encounter Alucard and his group. Naruto, with his ninja skills and strong will, might initially be seen as a threat by the Hellsing crew. But as they learn about his world and his fight for peace, they start to work together. Maybe Naruto tries to understand Alucard's vampiric nature and in turn, Alucard is intrigued by Naruto's chakra and jutsu. Together, they could face a new threat that is endangering both their universes.
I think Naruto and Integra would be a fascinating pairing. Integra is a strong and authoritative leader in the Hellsing world, and Naruto has his own leadership qualities within his ninja team. They could have some really interesting discussions about leadership, duty, and protecting their respective worlds. Also, Kakashi and Alucard would be a cool pair. Kakashi is known for his calm and collected nature, and Alucard's unpredictable and powerful presence would create a great dynamic. They could exchange knowledge about combat techniques and strategies from their different worlds.
One idea could be that the characters from Highschool DxD end up in the Hellsing universe. Maybe Issei and his friends are mistaken for new types of vampires due to their devil powers. They then have to team up with the Hellsing organization to clear their names and also fight against a common enemy that is threatening both the devil and human worlds in this combined universe.
The exploration of morality is also a big theme. When Hellsing crosses with a more 'pure - hearted' universe like 'My Little Pony' (in some very creative crossovers), it forces the characters to re - evaluate their views on good and evil. Alucard's brutal nature might seem out of place in the colorful world of ponies, and this contrast creates interesting moral discussions.
Immortality in Naruto - Hellsing fanfiction can bring a whole new level of storytelling. In Naruto, some characters are effectively immortal through means like the tailed beasts' chakra. In Hellsing, vampires are immortal. Combining these, a story could be about a character trying to find a way to end the immortality curse. Maybe Naruto uses his sage mode to understand the nature of vampiric immortality in Hellsing.
One possible crossover in Naruto and Hellsing fanfiction could be the sharing of powers. For example, Naruto's ability to use chakra could be combined with the vampiric powers in Hellsing. Maybe a character from Naruto gets turned into a vampire and has to deal with the new powers and the moral implications in the Hellsing universe.
In these crossovers, a common theme is the struggle between good and evil. Walter is often in a position where he has to decide which side he truly stands on in the Hellsing world. Another theme is the concept of time. Sometimes the fanfictions play with the idea of time, like Walter being sent back in time within the Hellsing universe or having to deal with time - related anomalies. And friendship is also a theme. His relationships with the characters in Hellsing, whether they develop into true friendships or remain strained, are often explored in these fanfictions.
There are quite a few. One interesting aspect is seeing how the moral and ethical concepts from both 'Bleach' and 'Hellsing' clash or blend. The storylines often involve the characters from both series having to team up against a common enemy that is even more powerful than what they usually face in their own respective worlds.
One common theme is the clash of different power systems. In Naruto, there are chakra and ninja techniques, while in Hellsing, there are vampires and powerful weapons. Fanfictions often explore how these different power systems interact when the two worlds meet.
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.