A cool plot could be that a group of vampires in New York are causing trouble, and they're not the typical vampires the Winchesters are used to. These vampires have some connection to the political and social world that the Blue Bloods characters are part of. The Winchesters and the Reagans have to figure out how to stop these vampires without causing a panic in the city. The Reagans can use their police and political influence, while the Winchesters bring their knowledge of vampire weaknesses and hunting techniques.
Well, perhaps there's a cursed object in the evidence room of the Blue Bloods precinct. The Winchesters hear about it through their hunter network and come to investigate. Along the way, they start to bond with the Reagan family members. Danny Reagan could be initially skeptical of the Winchesters' methods but gradually come to respect their skills in dealing with the supernatural. They could all work together to break the curse on the object before it causes more harm.
Perhaps the male OC is an undercover agent sent to investigate something within the police department in the Blue Bloods world. He has to navigate the complex relationships and hierarchies while keeping his true identity a secret. This could lead to a lot of tense and exciting moments as he tries not to blow his cover.
The OC could be a medical professional who treats Linda's family. Through this, Linda and the OC get to know each other. They start to rely on each other's expertise - Linda for her knowledge of the police world, and the OC for medical advice. This relationship could develop into a strong friendship or more as the story progresses.
One idea could be that the magical world of Harry Potter somehow intersects with the police - family - centered world of Blue Bloods. Maybe a wizard or witch commits a crime in the non - magical world, and the Reagan family from Blue Bloods has to deal with it while also trying to understand the magical elements involved. For example, a magical object is stolen and being used to cause chaos in New York City, and the Hogwarts students come to help solve the mystery, with the Reagans providing their non - magical expertise.
Castiel and Frank Reagan could also be an interesting pairing. Castiel with his angelic powers and Frank with his leadership and experience in the police force. Castiel might not understand all of the human politics and procedures that Frank deals with, and Frank might be initially shocked by Castiel's otherworldly nature, but they could find common ground in protecting the city from a major supernatural disaster.
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.