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.
Jason Todd's age varies depending on the specific comic storyline and continuity. But generally, he's often depicted as being in his late teens or early twenties.
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.
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 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.