In this fanfiction, perhaps a character from Hellsign finds themselves in the world of Supernatural and has to adapt to the different rules and monsters. They could offer their unique skills in dealing with demons and the occult, which might be different from what Sam and Dean are used to. This could lead to some interesting conflicts and alliances as they learn from each other.
A possible plot could be that there is a powerful artifact in the Hellsign universe that has the potential to either destroy or save both universes. The main characters from both Hellsign and Supernatural have to race against time and various villains who also want to get their hands on the artifact. Along the way, they face challenges that test their beliefs and abilities, and they might also discover some long - lost secrets about their respective worlds.
One idea could be a crossover where the characters from Hellsign have to team up with the Winchester brothers from Supernatural to fight a common supernatural threat that combines elements from both universes, like a demon that uses Hellsign - style magic.
Imagine a scenario where a Hellsign character who is used to working alone is forced to work with the Supernatural crew. At first, he is constantly butting heads with them, especially with Castiel, the angel. But as they go through a series of dangerous encounters, they begin to understand each other's motives and eventually become friends. Their interactions would be full of sarcasm, but also moments of genuine understanding and support.
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.
Another possibility is that a character from Glee, say Rachel Berry, discovers she has some sort of meta - human ability similar to the Flash. The Flash has to come and train her to control it while also dealing with the chaos that her new powers cause within the glee club dynamic.
Stitch could be accidentally teleported to Arendelle. Elsa, thinking he's a threat at first, freezes him. But then she realizes he's harmless. They then become friends and Stitch shows Elsa how to have more fun. He teaches her some of his crazy alien moves and Elsa in turn shows Stitch the beauty of her ice - made creations.
One idea could be that Alec, from the Malec universe, stumbles into Lucifer's realm while on a mission. He's initially confused and on guard. Lucifer, being the charming devil he is, tries to make a deal with Alec. But Alec, being true to his Shadowhunter values, refuses. This could lead to an interesting dynamic where Lucifer is intrigued by Alec's integrity and Alec is both repelled and fascinated by Lucifer's world.
One idea could be that the Overwatch heroes are transported to the Attack on Titan world. Tracer could use her time - traveling abilities to outmaneuver the Titans. Maybe Reinhardt's shield would be crucial in protecting the Survey Corps from Titan attacks.
Maybe the beautiful girl is actually a spy sent to gather information about Konoha. But as she spends time with Naruto, she's conflicted. Naruto, being his kind - hearted self, shows her the true spirit of Konoha. This changes her perspective, and she decides to abandon her spying mission and join Konoha. They then start a relationship based on mutual respect and new - found loyalty.
Another idea is that they meet in a time - loop scenario. Every time they meet, something different happens, and they have to figure out how to break the loop. It could be a really cool concept for a fanfiction.
One idea could be that the female Saiyan is from a different part of the universe. She's heard tales of Goku's power and comes to Earth to challenge him. But when she meets him, she realizes he's not just about power but also has a kind heart. So instead of fighting, they start training together.
One idea could be that the NCIS team is investigating a series of strange murders that seem to have a supernatural element. They end up calling in the Winchesters from Supernatural for help. The two groups initially clash due to their different methods but eventually learn from each other and solve the case together.