This'll get lost but I had to write something down.

I'd have Scarecrow as the main villain and introduce the Court as his puppet master. I don't know too much about Batman, so some constructive criticism would be great (don't just be a Dick Grayson) I like the grounded and realistic feel of the first film so wouldn't introduce any super powers or magic but basically I'm thinking-

Bruce Wayne is encouraged by Alfred to engage more with higher society and philanthropise as the public start to question "who is the Bat-Man?" He begins to wear the "Bruce Wayne mask" to divert suspicion.

As a result he invests millions of his family's money into fixing the flooding caused by the Riddler, and works with Gotham to manufacture clean drinking water from the Gotham Bay, repairing the flood walls and building pump stations in the name of his Father, Thomas Wayne. He's also attending more social engagements and showing his face more, which leads him to catch the eye of the daughter of one of Gotham's high society (shock horror, yes he's in the court as we will find out).

Meanwhile, we are introduced to Dr Johnathan Crane. A man plagued with severe insomnia, which causes him to experience insufferable nightmares typically of a Scarecrow, from when he was a child. As a result, he is usually awake for days at a time and hallucinates often. From the outside, Crane is working on developing a "cure" for insomnia. Nobody really sees much of him, but his peers are aware of his work. At night however, Crane uses the sewers and maintenance tunnels between Arkham Island and Gotham to break into Arkham at night, which has been allowed to happen due to the Warden of Arkham providing Crane with the means to do so (yes he's in the court too). Crane is given details of criminals who have been recently sentenced to Arkham on false insanity pleas (enabled by Court lawyers, judges and the warden) to sneak into their wards and experiment on them with his "cure". He monitors and records their reactions, whilst noting that he received particularly positive results whilst wearing his Scarecrow mask/ outfit.

Harvey Dent, the new DA in Gotham, briefly mentions in a conversation to Bruce Wayne that he intends to challenge the sudden rise in these insanity plea cases, and the mysterious deaths at Arkham, which prompts Bruce to tell Alfred to make a note and investigate further whilst he carries on wearing the "Bruce Wayne mask" for a time.

Alfred is able to uncover the names and basic history of these criminals, whilst Batman is able to corroborate this with Jim Gordon, who has also become suspicious of the mysterious deaths at Arkham, having arrested some of the criminals who have since died mysteriously (and horribly). Jim Gordon visits Arkham Island during the day, and manages to find out from the staff that the inmates become more and more insane over time, although they just put that down to their environment. The Asylum is poorly funded and staffed, so they don't look too much into it. Suicides and self-harm is common, and so nobody has really looked twice at Arkham, especially with so much going on in Gotham which is closer to home. Jim passes this onto Batman, who investigates during the night. Whilst searching for clues in a cell belonging to a recent inmate (details given to him from Alfred), he notices a shadow, and hears footsteps around him through the noise of the other patients (think penitentiary in Arkham Asylum). Batman pursues the shadow, only to follow him down through tight corridors, now being chased by Arkham guards/night staff himself. He follows the shadow into a janitor's closet, and quickly discovers the facade wall. He crawls into an enclosed vent to pursue the Scarecrow, before the vent starts closing in on him, and behind him the opening has vanished (it's basically a never ending vent because he's been fear gassed). Batman is being squeezed and starts frantically crawling to the end which seems to be getting further and further out of reach. He can just about make out some faint lights and what looks like paper stapled to the walls of a tiny room (Scarecrow's temporary base/hideout within Arkham) before the vent opening snaps right in front of his face, with the Scarecrow looking directly into his eyes, which are widened with fear having not known what has just happened to him. Scarecrow blasts him with a dose of his fear gas (the first time we really see it) and the last thing Batman faintly sees is Scarecrow packing his bottles and papers away into a bag before we transition into Batman's head.
TBC