A plot could involve a powerful ancient evil that has been sealed away in both universes. The seal is weakening in both the Hellsing world and the Supernatural world simultaneously. So, the protagonists from both sides must combine their knowledge and resources. Alucard from Hellsing could bring his immense power and immortality, while Sam and Dean Winchester could offer their knowledge of a wide variety of American - based supernatural entities and their hunting skills.
Maybe an OC devil who was once a human and was turned into a devil against their will. They could struggle with their new identity while still having some human emotions, which would create complex relationships with the Hellsing characters.
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.
One idea could be a battle between the forces of Hellsing and some unique threat from the Alexandee universe. Maybe the Hellsing crew has to deal with a powerful magic user from Alexandee who is causing chaos in their world.
One idea could be that the Addams Family, being the strange and macabre family they are, somehow get involved in the vampire - hunting activities of the Hellsing Organization. Maybe they see the vampires as kindred spirits in a way, but also recognize the danger they pose. Another could be a situation where a member of the Addams Family becomes a target of the vampires in the Hellsing universe, and the Hellsing crew has to work with the family to protect them.
A possible plot could involve Schrodinger being sent on a mission with Seras by the Hellsing Organization. During the mission, they face a powerful enemy that forces them to rely on each other's unique abilities. As they work together, a deeper understanding and an unexpected bond form between them. This could explore how their different natures, Schrodinger's quantum - like existence and Seras' vampiric nature, interact and complement each other in the face of danger.
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.
Maybe there's a rift in the multiverse that connects the two worlds. Integra discovers Danny's powers and tries to recruit him to help in the fight against the vampires. Danny, being a hero in his own world, is hesitant at first but realizes the importance of cooperation when he sees the scale of the vampire threat in the Hellsing universe. He could use his intangibility and flight to outmaneuver the vampires while Alucard and Seras provide the heavy - hitting attacks.
Alucard and Dean Winchester would be an interesting pairing. Alucard's dark and brooding nature, combined with his overpowering abilities, would contrast with Dean's more impulsive and human - centered approach.
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.
One idea could be creating an OC who is a new recruit in the Hellsing Organization. They could have a unique power, like the ability to control shadows, which adds an interesting dynamic to the battles against vampires. Another idea is an OC that is a vampire with a conscience, struggling between their blood - thirsty nature and the human values they still hold. This can create complex moral dilemmas within the Hellsing universe.