One way this could be developed is by having Naruto's abilities misused or corrupted in the Hellsing universe. Maybe some dark magic or experiment twists his chakra into a force that creates 'monsters'. For example, his shadow clone jutsu could be warped so that the clones turn into uncontrollable, monstrous entities. Another aspect could be his connection to the Nine - Tails. If the Hellsing organization were to try and harness the power of the Nine - Tails, it could go horribly wrong and lead to the creation of new and terrifying monsters.
It would mean a major shift in the power dynamics. Naruto's monsters could either be a new threat to the Hellsing organization or a new weapon they could try to control. For example, if the monsters have Naruto's ability to grow stronger quickly, they could pose a significant challenge.
In the 'Naruto Father of Monsters' fanfiction, perhaps Naruto discovers he has an ancient bloodline that is connected to some mythical monsters. This newfound connection could put him at odds with the Hidden Villages as they fear the power he might unleash. He then has to fight to protect his new family of'monsters' while also trying to keep the peace in the ninja world. It could also involve him teaching his'monster' children the ways of the ninja, which would be really interesting to read about.
In this fanfiction, the 'Father of Monsters' might be an ancient wizard who has been in a deep slumber for centuries. He was awakened by some disturbance in the wizarding world and, feeling angry about the changes that have occurred, decides to create monsters to bring back what he believes was the 'true' order of the wizarding world. His knowledge of old magic gives him the ability to create extremely powerful and dangerous monsters that no one has ever seen before.
The main antagonists could be some of the traditional ninja clans who see Naruto's connection to the monsters as a threat to their power structure. They might fear that the monsters will disrupt the balance of power in the ninja world.
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.
There could be a flying monster with razor - sharp claws. This kind of monster can swoop down on villages unexpectedly, making it a fearsome adversary for the ninja. It would force the ninja to develop new strategies to deal with aerial threats.
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.