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.
Well, for the characters' interaction, focus on their goals. Batman's goal is to protect Gotham, and the Winchesters' is to hunt down the supernatural. When they interact, these goals can either clash or align. For instance, if there's a supernatural threat in Gotham that the Winchesters are after but Batman doesn't fully understand the nature of it yet. There will be some tension as Batman tries to maintain his control over the situation in his city. But as they communicate, they can find common ground. You can also use their different ways of speaking. Batman is more formal and the Winchesters use a lot of slang, which can create interesting dialogue exchanges.
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.
How about Gollum from Lord of the Rings? It seems really odd at first, but think about it. Gollum's sneaky nature and knowledge of dark places could be useful in Arkham Asylum. Batman could try to control Gollum's greed and use his skills to navigate the more dangerous parts of the asylum. And Gollum might be fascinated by Batman's shiny gadgets. It would be a very strange and interesting dynamic.
Another one worth checking out is 'The Caped Crusader in RWBY'. It delves deep into how Batman's code of ethics might change or stay the same in the RWBY universe. It also has some great fight scenes where Batman uses his martial arts skills against Grimm, all while trying to understand the complex politics of Remnant.
One great Batman crossover fanfiction is 'Batman and the Avengers: A Dark Alliance'. It combines the gritty world of Batman with the super - powered heroes of the Avengers in an exciting way. The story has some great action scenes and interesting character interactions, like Batman's tactical mind clashing with Iron Man's high - tech approach.
One of the best elements is seeing Batman interact with characters from different universes. For example, when he teams up with Superman, we get to see how their different moral compasses and powers play off each other. Another element is the new villains or threats that are created by the crossovers. It adds a fresh perspective to Batman's world.
Another possibility is a scenario where Voldemort decides to expand his reach into the non - magical world and Batman becomes aware of this threat. He would then enter Hogwarts undercover to learn about magic and form an alliance with the Order of the Phoenix. The contrast between Batman's brooding, non - magical detective nature and the magical world would make for an interesting story.
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.