Some of the other inmates like Two - Face could also be antagonists. Two - Face's split personality could make him unpredictable. He might see Jason as an easy target or as someone who is interfering with his own plans within the asylum, and thus constantly try to attack or undermine him.
The Joker could be a main antagonist. Given his history with Jason Todd, he might try to further torment him in Arkham. He could play mind games with Jason, trying to break his already fragile mental state.
The asylum staff themselves could be antagonists. If there is a corrupt system in Arkham, they might be trying to keep Jason trapped for their own purposes. They could withhold treatment or resources from him to make his stay more difficult.
One possible plot could be Jason Todd's struggle to maintain his sanity within Arkham Asylum. He might be haunted by his past traumas and face off against the various villains locked up there. Maybe he's trying to find a way to escape while also dealing with the asylum's staff who may or may not have their own hidden agendas.
The Joker would definitely be a major antagonist. His chaotic and unpredictable nature would pose a huge threat to Danny. He'd likely be intrigued by Danny's powers and try to use them for his own amusement or evil schemes. Another one could be the Penguin. He might see Danny as a new obstacle in his plans within Arkham or as something he could sell to the highest bidder, perhaps to other villains outside the asylum who are interested in Danny's unique abilities.
Well, there's a fanfiction called 'Jason Todd: Beyond the Arkham Knight'. In this story, Jason tries to find his own identity separate from the labels of Arkham Knight and Robin. It has some really cool action scenes and also explores his friendships and enmities in Gotham. And then there's 'Arkham Knight's New Path' which is about Jason forging a new, more positive path for himself in Gotham.
In Arkham Knight fanfiction, a Jason Todd OC might have unique abilities or a backstory that ties into the Arkham Knight's world. Maybe the OC has a connection to the criminal underworld in Gotham that Jason can use to his advantage in his fight against crime. It could also be that the OC is a scientist who helps Jason with some of the tech - related challenges he faces.
I really like 'Jason Todd's Return: Arkham Knight's Hidden Story'. This fanfic focuses on Jason's role in the Arkham Knight events from a different perspective. It shows how he might have been feeling during all the chaos, and it gives a more sympathetic view of his actions. It's a shorter read but still packs a punch in terms of character exploration.
One of the main reasons is the potential for character growth. Jason Todd has had a rough journey, and in Arkham Knight fanfiction, we can see him evolve. Maybe he overcomes his hatred for Batman, or he becomes a better hero. Also, the Arkham Knight setting is so rich. The dark and gritty atmosphere of Gotham in that universe provides a great backdrop for stories about Jason. There are so many elements like the other villains, the technology, and the city's layout that can be used to create interesting and engaging fanfiction about him.
When creating a Jason Todd OC for Arkham Knight fanfiction, start with a strong backstory. Consider what events in their past led them to cross paths with Jason. It could be something as simple as growing up in the same rough neighborhood.
The Joker could be the main villain. He might see Harry as a new plaything and constantly torment him with his cruel pranks and mind games, making Harry's stay in Arkham a living nightmare.
One possible antagonist could be an ancient and powerful Titan who has escaped from Tartarus. This Titan is bent on revenge against the gods and sees Percy and Jason as the key to getting to the Olympians. He uses his dark magic to create all sorts of disasters and tries to turn Percy and Jason against each other. Another could be a group of rogue demigods who have turned to the dark side. They envy Percy and Jason's status and friendship and try to undermine them at every turn.
The popularity of 'Batman: Arkham Knight' Jason Todd fanfiction lies in the character's complexity. Jason has a tragic backstory - being killed and then resurrected. Fanfiction writers can explore his journey of redemption or revenge. For example, in many stories, we see him struggling with his identity, which is a very engaging theme.