One plot idea could be that Aziraphale and Crowley from Good Omens get dragged into a case that the Winchester brothers are investigating in Supernatural. Maybe it's a demon that's a bit too powerful for Sam and Dean, and they need the help of the angel and demon duo who have a different approach to dealing with the supernatural. Another idea is that the two universes' versions of Heaven and Hell start to merge, and characters from both shows have to figure out how to stop it while also dealing with their own personal issues.
Well, a really cool plot could be that the Antichrist from Good Omens ends up being a key figure in a prophecy in the Supernatural universe. Sam and Dean initially think he's just another evil they need to stop, but Aziraphale and Crowley try to convince them that he's not all bad. The four of them then have to go on a journey to find a way to use the Antichrist's power for good in the Supernatural world, all while being chased by various angels and demons from both universes.
A plot could involve Castiel from Supernatural meeting Aziraphale. They could have an interesting conversation about their different experiences as angels. Castiel is so used to the strict hierarchy in his Heaven, while Aziraphale has been bending the rules for a long time. Meanwhile, Crowley could be teaching Dean some of his more devious ways to deal with demons, much to Sam's dismay. And perhaps there's a powerful artifact that has the power to destroy both the Good Omens and Supernatural worlds, and they all have to team up to find and protect it.
One plot idea could be that the characters from Good Omens and Homestuck find themselves in a shared alternate universe. The angels and demons from Good Omens might be quite confused by the strange and complex world of Homestuck, while the Homestuck characters could be intrigued by the celestial beings. For example, Crowley could team up with a Homestuck troll to outwit some overzealous angels. Another idea is a crossover where they have to solve a mystery that spans both their universes. Maybe a powerful artifact has been stolen and its pieces are scattered across both worlds, forcing the characters to cooperate despite their differences in nature and origin.
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.
One interesting pairing could be Dean Winchester and Crowley. Dean has a lot of experience dealing with demons in the Supernatural world, and Crowley has his own unique charm and deviousness. Their interactions would be full of witty banter and power plays. Another pairing could be Sam Winchester and Aziraphale. Sam is the more intellectual of the Winchester brothers, and Aziraphale is an angel with a vast knowledge of the cosmos. They could have deep conversations about morality and the nature of good and evil.
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.
Well, a cool plot could be that an ancient, powerful entity from the Supernatural world is awakened in Smallville. Clark discovers it while trying to protect the town. He realizes he can't handle it alone and contacts Bobby Singer through some means. Bobby then sends Sam and Dean to help. They initially have a hard time trusting Clark because of his superpowers, but soon realize they need his help. As they battle the entity, they discover it has ties to an old, forgotten lore that involves both the kryptonite on Earth and a powerful demon they once fought. They have to combine their knowledge and powers to defeat it.
Perhaps there could be a crossover where the Endbringers in 'worm' are actually manifestations of some ancient supernatural evil. And a group of supernatural hunters from another dimension come to Brockton Bay to fight them, teaming up with local parahumans.
One idea could be that the Winchesters end up in Star City and team up with the Arrow to fight a demon that is using technology to cause chaos. Maybe the demon has possessed some of the tech - savvy villains in the Arrowverse.
One idea could be that the Winchesters are hunting a meta - human in Central City who has powers similar to a demon. The Flash team tries to stop them from killing the meta - human, thinking they are just bad guys. Another could be a time - travel situation where Barry accidentally travels to a world where the Supernatural events are real and has to team up with Sam and Dean to get back home. A third idea is that a powerful witch from the Supernatural universe uses her magic to create chaos in Central City, and the Flash and his friends need the help of the Winchesters to defeat her.
One interesting plot idea could be that the Flashpoint event somehow creates a rift that pulls characters from Supernatural into the Flashpoint universe. For example, Sam and Dean Winchester could be chasing a demon that escapes into this new, altered reality. They'd have to deal with the different laws of physics and the presence of superheroes like Flash. Another idea is that a powerful supernatural being from the Supernatural world causes the Flashpoint event in the first place, perhaps as part of a larger plan to reshape all of existence. And a third idea could be that a character from Flashpoint, like Iris West, has some latent supernatural ability that is awakened by the presence of the Winchester brothers, and they have to figure out how to control it before it causes chaos in both their worlds.
One idea could be that the Avengers are facing a supernatural threat that they can't understand. So they seek the help of the Winchesters from Supernatural. The Winchesters introduce them to the world of the supernatural, like demons and ghosts. And together they combine their skills, the Avengers' superpowers and the Winchesters' hunting knowledge to defeat the threat.