Definitely Dean Winchester and Barry Allen. Dean has his tough - as - nails hunter attitude and Barry is the optimistic speedster. They could have some great banter while on a mission together. For instance, Dean might be constantly amazed at Barry's speed but also make fun of how he doesn't know how to handle a real weapon like a gun.
Iris West and Castiel. Iris is a strong - willed journalist and Castiel is an angel with a lot of knowledge. Iris could be interested in getting the story of the supernatural from Castiel, and Castiel could be intrigued by Iris's determination. They could form an unlikely friendship where Iris helps Castiel understand human emotions better, and Castiel protects Iris from any supernatural threats that come her way.
Cisco Ramon and Sam Winchester would be an interesting pair. Cisco is the tech - savvy genius in The Flash and Sam is the more research - oriented Winchester. They could work together to figure out the scientific and supernatural aspects of whatever threat they are facing. Cisco might be able to use his tech knowledge to enhance Sam's research tools, while Sam could introduce Cisco to some ancient texts about the supernatural.
Iron Man and Castiel would be an interesting pair. Iron Man with his high - tech gadgets and Castiel with his angelic powers. Iron Man could be intrigued by Castiel's ability to teleport and heal, and Castiel might be fascinated by Tony's technology. They could work together to fight a threat that combines technology and the supernatural, like a group of demons that are using stolen Stark tech.
Edward Cullen and The Flash would make for an exciting pair. Edward has his vampire powers, including super - strength and enhanced senses. When combined with The Flash's super - speed, they could be an unbeatable team. Imagine them racing against each other at first, testing each other's limits, and then realizing they can achieve more together. Also, Edward's brooding nature and The Flash's more upbeat and positive attitude would create a great contrast in the story.
Goku Black could be an interesting character to pair with Gogeta. Their power levels are both high and their personalities are quite different. Goku Black is more of a villainous character, so the dynamic between him and Gogeta, who is a hero, would be full of tension. Another interesting character could be Saitama from One Punch Man. The contrast between Gogeta's complex power - up system and Saitama's one - punch - kill ability would make for an exciting crossover. And then there's Naruto. His ninja skills and determination paired with Gogeta's power would create a unique combination.
One idea could be that a supernatural entity from the Supernatural world somehow ends up in Central City and starts causing chaos that only the Flash's speed can help contain. For example, a demon that can possess multiple people at once and the Flash has to use his speed to track down all the possessed bodies before the demon can complete a dark ritual.
Wonder Woman is another great choice. Her strong moral code and powerful abilities would be an interesting counterpoint to Deadpool. He might be in awe of her at first, and then start trying to crack jokes and get on her nerves, but they could also form an unlikely friendship as they fight together against a common enemy.
Spiderman and Batman would be really interesting. They are both intelligent and resourceful. Batman has his gadgets and detective skills, while Spiderman has his super - strength and web - slinging. They could learn a lot from each other.
Castiel and Danny would be an unexpected but cool pairing. Castiel is an angel with his own unique powers and view of the world. Danny's half - ghost nature would be something completely new to Castiel. They could have some really deep and strange conversations about the nature of life, death, and the afterlife. And when it comes to fighting enemies, their combined powers could be really effective.
Definitely Percy and Dean. Percy has his quick - thinking and his water powers from being a son of Poseidon, while Dean is the tough, street - smart hunter. Their different personalities and skills would create an interesting dynamic. They could learn from each other, like Percy teaching Dean about Greek mythology and Dean showing Percy some of his hunting tricks.
Arya Stark and Castiel. Arya is a skilled assassin with a thirst for revenge. Castiel is an angel who often struggles with understanding human emotions. Their interactions could be really fascinating. Castiel could help Arya understand the deeper spiritual side of her revenge, while Arya could teach Castiel about the importance of personal vendettas in the context of the cut - throat world of Westeros.
In some fanfictions, there might be a situation where the Flash's speed force is somehow connected to the supernatural world in Supernatural. Maybe it attracts the attention of angels or demons who want to use it for their own purposes. This could lead to a complex plot where the Flash has to deal with both his own villains and the otherworldly beings from Supernatural.