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.
A great plot could involve the Angels and Demons from Supernatural getting involved in the Flashpoint timeline. Maybe the Angels see the altered reality as a threat to their plans and try to restore the original timeline. The Demons, on the other hand, might see it as an opportunity to gain more power. The Flash and his team would be caught in the middle of this supernatural battle. Or, what if a spell from Supernatural accidentally sends Barry Allen into the world of Supernatural? He'd have to find his way back while dealing with all the monsters and hunters there. Also, we could have a crossover where the anti - matter wave from Flashpoint somehow interacts with the Darkness from Supernatural, creating a new and dangerous entity that both teams have to face together.
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 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.
One idea could be that the characters from Arrow get transported to the Supernatural universe. Oliver Queen could team up with the Winchesters to hunt a demon that has somehow made its way into Star City. Maybe the demon is using some of the technology in Star City to create chaos, and they have to combine their skills - Oliver's archery and combat skills with the Winchesters' knowledge of the supernatural - to stop it.
There could be a Flashpoint Crossover Fanfiction that focuses on the Flashpoint villains crossing over into a more stable DC universe. The villains would be out of their element, and seeing how they adapt and cause chaos in a new world would be really engaging. Their motives might change as they encounter new challenges and heroes.
One idea could be that the Winchester brothers from Supernatural end up in the world of The Walking Dead. They could use their hunting skills to fight off the walkers. Maybe they'd discover that the zombies are somehow related to a demonic presence and try to find a way to stop it.