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.
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.
It enhances the series by adding more content. Simple as that. More stories mean more gameplay time and more things to discover about the Arkham world.
The main events start with Batman taking the Joker to Arkham. Then the Joker takes over. Batman battles various villains like Scarecrow who uses his fear toxin. Another event is Batman's encounter with Poison Ivy and her plants. He also has to face Killer Croc in the sewers of the asylum. And of course, the big event is trying to stop the Joker from using the Titan formula on the inmates to create a super - villain army.