Castiel's stoic and somewhat naïve personality in Supernatural would contrast with the more worldly and sly characters in Good Omens. However, his loyalty and sense of duty could be something that the Good Omens characters would admire. And in return, the Good Omens crew could teach Castiel to loosen up a bit and not be so rigid in his thinking.
The powers of the Supernatural characters, like Sam's psychic abilities and Dean's combat skills, would contrast with the more otherworldly powers of the Good Omens characters. Aziraphale's ability to perform miracles and Crowley's demonic powers would be on a different scale. For example, Sam's psychic visions might be able to predict events that Aziraphale and Crowley are involved in, but they might not fully understand how he gets these visions at first.
Well, Shawn from Psych is really outgoing and a bit of a show - off, while the BAU agents in Criminal Minds are more reserved in their own ways. In fanfiction, it can be really interesting to see how Shawn's antics either annoy or amuse the Criminal Minds characters. For example, Derek Morgan might be initially frustrated by Shawn's non - traditional methods but then come to respect his insights.
Another great storyline could involve the prophecies in Good Omens intersecting with the apocalyptic events in Supernatural. The characters from both would then be scrambling to prevent or at least manage the impending doom. The Horsemen from Supernatural might interact with the Horsemen - like figures in Good Omens, leading to a battle of wills and powers that would keep readers on the edge of their seats.
The New Girl characters have very distinct personalities. Jess is the idealistic dreamer, always seeing the best in people and situations. In a crossover with Percy Jackson, this could lead her to trust the wrong characters at times, but also to befriend some of the more misunderstood creatures. Nick's laziness might be a problem when it comes to the physically demanding tasks in the Percy Jackson world, but his loyalty to his friends would shine through. Schmidt's competitiveness could either make him a great ally in battles or cause him to make hasty decisions. And Winston's love for animals could potentially lead him to form an unlikely bond with some of the mythical beasts in the Percy Jackson universe.
The Winchesters are tough and used to a more violent, hands - on approach. Mulder is more cerebral and curious. When they interact, it can be interesting. For example, Dean might be skeptical of Mulder's theories at first, but then see the value in his way of thinking. Scully, on the other hand, might be frustrated by the Winchesters' lack of scientific method, but come to respect their instincts.
One way to create an engaging plot for this crossover fanfiction is to focus on the characters' reactions to the new world they find themselves in. Imagine Dean Winchester suddenly in the middle of London and being confronted by an angel and a demon who don't quite act like the ones he's used to from his own world. His confusion and initial mistrust could lead to some interesting dialogue and action scenes. Sam, on the other hand, might be more open - minded and start asking questions about the Good Omens universe. Meanwhile, Aziraphale and Crowley are trying to figure out who these strange American hunters are. This interplay between the characters from different universes can drive the plot forward.
One idea could be that Sam and Dean from Supernatural get thrown into the world of Good Omens. They team up with Aziraphale and Crowley to stop a demon from opening a portal that would unleash all kinds of chaos. The Winchesters' hunting skills combined with the angel - demon duo's knowledge of their own universe could make for an exciting adventure.
The werewolves in Twilight have their own set of abilities like enhanced strength and pack telepathy. Inuyasha also has his encounters with wolf demons. In a fanfic, the werewolves could team up with Inuyasha against a common enemy, perhaps some powerful demon that has made its way into the Twilight universe. Their combined strength and abilities could create a really dynamic and action - packed story.
Their personalities are quite different but complementary. Hermione's love for knowledge could inspire Viktor to learn more about the academic side of magic. Viktor's courage and boldness in the face of competition might make Hermione more confident in her own abilities outside of just academics. They would respect each other's traits and use them to build a strong relationship.
Well, for Tony Stark, he might become a bit more laid - back around the Guardians. The Guardians' carefree attitude could rub off on him. He may start to make more jokes and not be as uptight about his technology and plans.