One idea could be exploring Scarecrow's backstory in more depth. Maybe show his early days in Arkham Asylum, how he was treated by the staff and inmates, and what led him to become the terrifying Scarecrow. Another plot could involve Scarecrow creating a new and more powerful fear toxin that not only affects Batman but also the citizens of Gotham, and Batman has to race against time to find an antidote while dealing with his own fears.
For Scarecrow's character description, start with his physical presence. He moves in a jerky, almost inhuman way, like a puppet on strings. His hands are bony and claw - like. His eyes, which can be glimpsed through the holes in the sack, are filled with a manic glint. In terms of his character, he's obsessed with fear. He sees it as a tool, a weapon, and a form of art. He has no qualms about using it on anyone, whether they're a criminal or an innocent.
One great 'Batman Arkham Asylum' fanfiction is 'The Dark Knight's Redemption'. It delves deeper into Batman's psyche as he battles his inner demons while trying to escape the asylum. The author does a great job of capturing the dark and gritty atmosphere of the game.
The plot mainly revolves around Batman's encounters with Scarecrow. Scarecrow uses his fear toxins to spread chaos in Arkham. Batman has to face his own fears while trying to stop Scarecrow from causing more harm to the city.
First, know the source material well. Understand the characters, the setting of Arkham Asylum, and the overall mood of the game. For example, Batman is a complex character with a dark past, so make sure to capture that in your writing.
A more complex fanfiction could involve all of Team RWBY in Arkham Asylum. Blake could use her stealth skills to move around unnoticed, Yang's strength would be useful in physical confrontations. They could be there because some powerful enemy has lured them there, thinking it's a place where they'll be weakened. But together with Batman and some of the inmates who might side with them (like Harley Quinn who has a complex sense of morality), they could turn the tables on the enemy.
The plot involves Batman being lured into Arkham Asylum by the Joker. Once inside, he discovers that the inmates have taken over and he must fight his way through them. There are also sub - plots about the history of the asylum and the mental states of the villains, making for a complex and engaging story.
One idea could be that Naruto is somehow transported to Arkham Asylum. He might initially be confused by the strange new world, but his ninja skills could come in handy. For example, he could use his speed and agility to avoid the traps and villains in the asylum. Maybe he teams up with Batman, who is intrigued by Naruto's unique abilities.