One idea could be a story where a werewolf from the werewolf lore gets caught in the middle of a Supernatural's Winchester brothers' hunt. Maybe the werewolf has information about a powerful demon they are chasing.
A popular character could be a half - werewolf, half - hunter. This character would be constantly torn between their two natures, and in a crossover, they could be a bridge between the werewolf community and the hunters in Supernatural.
Another great concept is a crossover where Eleven's powers are mistaken for some kind of witchcraft by the Supernatural universe. Sam and Dean initially think she's a threat but then realize she's actually a powerful ally against a common enemy. They could be facing a powerful being that can manipulate time and space, and Eleven's telekinetic and telepathic abilities are crucial in defeating it.
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.
I would recommend 'The Werewolf's Secret'. This fanfiction focuses on a werewolf who has to hide his true identity in a world full of supernatural hunters. It's full of action and suspense, and the relationship between the werewolf and the other supernatural beings in the story is quite complex and interesting.
One great 'Twilight werewolf crossover fanfiction' is 'Twilight Meets the Pack from Another World'. It combines the allure of the Twilight werewolves with an exciting new world concept. The story delves into how the Twilight werewolves interact with characters from a different universe where werewolves have different powers and social structures. It's really interesting to see how the Twilight characters adapt to this new situation.
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.
In a crossover fanfic, the angels and demons from Supernatural could be involved in The Walking Dead universe. For example, an angel might be sent to Earth to stop the spread of the undead, believing it's part of a grand plan to save humanity. Meanwhile, the demons could be taking advantage of the chaos, making deals with desperate survivors. The Winchester brothers would have to navigate this new dynamic while also dealing with the walkers.
A cool plot could be a time - travel crossover. The 'Supernatural' crew travels to a time when Star City was in its early days. They meet a young Oliver Queen and end up changing some events that have a ripple effect on both their universes. This could lead to new alliances and new enemies. Also, a powerful witch from 'Supernatural' could put a spell on Star City that only the combined efforts of the two groups can break.
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.