Here are the ideas for this that God blessed me with:
Over 10 years after the first movie, Arthur has been declared sane and released from Arkham, back onto the streets of Gotham.
Meanwhile Bruce has returned and, filled with rage at the death of his parents, hates Arthur and sees him as the cause for all his pain.
Arthur feels psychological pressure (from those who treat him with disgust and contempt and those who adore him for committing those acts of murder), and from the understanding that in the years since this event happened, he, by the large public, has been forgotten about, this enraging him. His art therapy manifesting as Bruce Wayne, with blood on his face (that was splattered on it when his parents were shot), blood that increasingly across his art work takes over Bruce's face more and more, visually becoming his face. Arthur even seeing brief hallucinations of Sophie, going to her apartment eventually, to find out that she no longer lives there and is thought to have taken off with her daughter without informing anyone, due to the danger of the Gotham riots (no one knowing for sure, to maintain the uncertainty of the events of the first movie). His paranoia grows when he feels he's being followed by some mobsters. Arthur goes missing around this time.
This all culminating in a confrontation between Batman and Red Hood One.
