Well, you could create a fanfiction where the dementors in the Harry Potter universe are somehow related to the Scarecrow's fear toxin in Arkham Knight. Picture a scenario where the heroes from both worlds have to figure out how to combat this new, combined threat. Also, a story about Hermione using her intelligence to analyze the technology in Arkham Knight's Gotham and applying her magic in innovative ways to help could be really cool.
A really interesting fanfiction concept could be that Voldemort teams up with the Joker. In this story, the Joker has somehow discovered a way to use magic, perhaps through a cursed artifact. Harry and Batman then have to put aside their differences and work together to stop this unholy alliance. The contrast between the magical spells and the high - tech gadgets would make for a very exciting and unique story.
Another plot could involve Weiss using her family's resources to create technology similar to what's in Arkham Knight's world to combat a new type of Grimm. And the Arkham Knight himself might be interested in this tech and try to steal it, leading to an interesting conflict and alliance between Weiss and the other RWBY characters against him.
One idea could be exploring how the Knightfall event affects Batman's relationships with his allies in the Arkham Knight universe. Maybe some of them start to question his methods more deeply.
One interesting plot idea could be a fanfiction where the Joker somehow comes back to life in Arkham Knight. Maybe he was only in a coma and wakes up with a new, even more deranged plan to take over Gotham. He could start by manipulating the inmates in Arkham again, creating chaos from within the asylum.
In this fanfiction, it could be interesting if Catwoman discovers a secret about the Arkham Knight that Batman has overlooked. She could use this knowledge to her advantage, either to make a deal with the Arkham Knight or to help Batman. For example, she might find out that the Arkham Knight has a weakness related to a certain location in Gotham that was important to his past. Catwoman could then lead Batman there to take down the Arkham Knight.
One idea could be a story where Harry discovers he has an ancient dragon knight heritage. He then has to learn the ways of the dragon knights in secret while still dealing with Voldemort. Another could be that a dragon knight from the past is reincarnated as Harry, and he has to use his past skills to fight new threats at Hogwarts.
One interesting Arkham Knight fanfiction could be about the backstory of some minor characters in the game. For example, a story that dives deep into the life of a thug before they got involved with the criminal underworld of Gotham. It could show how the chaos in the city affected ordinary people and led them astray.
Well, a great plot idea might be delving deeper into the relationship between Batman and his rogues gallery in the aftermath. For example, how the villains react to Batman's continued presence or absence. Some might see it as an opportunity to take over Gotham, while others might be secretly glad he's still around to keep a certain order. Also, exploring the technological fallout. With all the advanced tech used during Arkham Knight, there could be some unforeseen consequences like tech falling into the wrong hands or new, unexpected uses for it.
One interesting story could be about Batman's internal struggle with the darker side of Gotham while facing the Arkham Knight. It might explore how he overcomes his doubts and fears in a more personal way than in the game.
A great plot idea for this fanfiction could be that Harley Quinn is haunted by the memories of her time with the Joker during the Arkham Knight scenario. Batman, noticing her distress, tries to help her. As they interact more, Harley starts to see Batman in a new light and decides to assist him in his fight against the Arkham Knight. Along the way, they face many challenges, like dealing with Harley's old gang who don't trust her new alliance with Batman, and also the Arkham Knight's devious traps. This would make for a complex and engaging fanfiction story.
First, decide on the main characters from both universes that you want to include. For example, you might choose Harry and Batman. Then, think of a starting point, like a mysterious portal opening between Hogwarts and Gotham City. Just start writing and let the story unfold from there.