Perhaps a spell gone wrong in 'Supernatural' accidentally opens a portal to the 'Grimm' world. Sam and Dean have to navigate this new world filled with Wesen while also trying to find a way back home. They soon discover that some of the monsters they hunt in their world have counterparts in the 'Grimm' world with different powers.
A really cool storyline might involve a powerful object or spell that links the two shows. For instance, a magical artifact in 'Supernatural' that can also affect the Wesen in 'Grimm'. The characters from both shows would have to race against time to find and control this object before it falls into the wrong hands. It would be a high - stakes adventure that combines the best elements of both shows, like the family - centered themes in 'Supernatural' and the unique fairy - tale inspired world of 'Grimm'.
The characters can interact through a cross - dimensional rift. For example, Dean Winchester might stumble into the 'Grimm' world and be mistaken for a Wesen at first. Nick Burkhardt would then have to figure out who he really is and they could start working together against common threats.
Writers can start by researching the lore of both shows thoroughly. They need to understand the rules of the 'Supernatural' world, like how angels and demons operate, and also the nature of the Wesen in 'Grimm'. Then, they can find common elements to build on. For example, both shows deal with creatures that are hidden from the normal human world. Writers can use this shared concept to create a scenario where the characters from both shows meet. Maybe a Wesen is causing trouble that has 'Supernatural' implications, so the 'Supernatural' characters get called in, and the 'Grimm' characters are already on the case.
One popular Grimm Supernatural fanfiction is 'The Crossroads of Worlds'. It combines the unique elements of both shows in an exciting way, with the characters from Grimm interacting with those from Supernatural. Another great one is 'The Hunters' Alliance' which focuses on a team - up between the hunters in both universes to fight a common, powerful enemy.
One idea could be that the NCIS team is investigating a series of strange murders that seem to have a supernatural element. They end up calling in the Winchesters from Supernatural for help. The two groups initially clash due to their different methods but eventually learn from each other and solve the case together.
One idea could be that Lucifer from 'Lucifer' enters the world of 'Supernatural'. Maybe he teams up with the Winchester brothers to fight a common enemy, like a new type of demon that is causing chaos in both their realms. He could use his devilish charm and powers in a way that the Winchesters are both skeptical and intrigued by.
One popular element is the mixing of different magical systems. For example, when crossing over with Harry Potter, the magic spells from the wizarding world and the fairy - tale magic in the Sisters Grimm universe interact in interesting ways. Another is the character interactions. When the Sisters Grimm characters meet characters from other franchises like those in Once Upon a Time, their different personalities and motives create engaging storylines.
One great Merlin - Supernatural crossover fanfic is 'The Hunter and the Warlock'. It combines the magic of Merlin's world with the demon - hunting aspect of Supernatural in a really interesting way. The characters from both shows interact in a believable manner, making for an exciting read.
One idea could be that the Winchester brothers from Supernatural are investigating a series of strange murders in Gotham City. They cross paths with Batman who initially is suspicious of them. Eventually, they realize that a demon is behind the killings and they team up. Batman provides his detective skills and combat abilities while the Winchesters use their knowledge of the supernatural to defeat the demon.