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.
Well, a cool plot could involve a rift opening between the Supernatural world and the Arrowverse. Characters from both universes start to swap places randomly. Sam and Dean could find themselves in Central City and have to deal with metahumans while Barry Allen ends up in a haunted forest. They then have to find a way to close the rift while also dealing with the threats in each other's worlds.
A possible plot for this crossover fanfiction is that an ancient, powerful magic has been unleashed in the Arrowverse which is connected to the supernatural realm. The Winchester brothers are called in to investigate. They meet Oliver Queen and his team, and together they discover that a powerful witch from the Supernatural world is trying to take over the Arrowverse cities. They must combine their skills, the Winchesters' knowledge of the supernatural and the Arrow team's combat and strategic skills to stop her.
One idea could be a story where characters from different Arrowverse shows team up to fight a new, powerful villain from a different dimension. For example, Oliver Queen from 'Arrow', Barry Allen from 'The Flash', and Kara Danvers from 'Supergirl' could combine their skills and powers to take on this threat.
Maybe the Flash could visit the MCU. He could race against Quicksilver. It would be a really cool showdown of speedsters from different universes. And then the Flash could get involved in an MCU - level threat, like Thanos' invasion but in a different timeline. He could use his speed to try and steal the Infinity Gauntlet from Thanos, creating a whole new plotline for a fanfiction.
A cool concept could be a crossover where the technology in Gotham and the magic in Star City somehow get mixed up. Batwoman, being more used to high - tech gadgets, and Arrow, who has some experience with magic - related threats, have to combine their skills to set things right. For example, a device that was supposed to enhance Batwoman's suit ends up being enchanted by magic from Star City, and they need to figure out how to fix it and stop the villain who caused the mix - up.
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 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.
One idea could be that the Charmed Ones have to team up with the Winchesters to stop a powerful demon that is causing chaos in both their worlds. Maybe the demon has a spell that can break the magical protections of the Halliwell Manor and is also targeting hunters. Another could be a crossover where Phoebe's premonition power combines with Sam's psychic abilities to foresee a major apocalypse event that they must prevent together.
One idea could be that a powerful witch from 'The Originals' world accidentally opens a portal to the 'Supernatural' world. Sam and Dean Winchester are drawn through it and end up in New Orleans. There, they team up with the Mikaelson family to fight a common enemy, like an ancient and powerful demon that is causing chaos in both their worlds.