All of which would be on brand for Phillips, who not so subtly remade in all but name classic noirs and psychological dramas from Martin Scorsese’s early career, namely Taxi Driver (1976) and The King of Comedy (1982) when he co-wrote the first Joker screenplay.
So Arthur and Harley becoming Mickey and Mallory seems very on brand for Phillips’ recontextualization of notorious late 20th century cinema in early 21st century IP garb. However, both that fact and the National Institutes of Health’s definition for “folie à deux” makes us wonder if there is another direction the film could be going in entirely.
According to the NIH (via NCBI), “Folie à deux is defined as an identical or similar mental disorder affecting two or more individuals, usually the members of a close family.”
It can refer to people who are family. Or at least think they are. And if you recall in the original Joker movie, before calling himself Joker, Arthur Fleck presented himself to a young Bruce Wayne as his long lost brother. This is an idea (or delusion) that had to become quite horrible for Bruce after Arthur’s later anarchic rage against the machine caused a mob to murder his parents.
One of the very last things we see in Joker is Arthur, now devoid of his makeup, in a psychiatric ward laughing to himself at the thought of rich boy Bruce Wayne being orphaned by the have-nots of Gotham City. This is obviously a thread of great significance not only to this version of Batman and Joker mythos, but to the themes Phillips wanted to explore about the cruelties of a society where the wealthy practically get away with murder… until the Joker and his forces murder them.
So while pivoting to a Joker and Harley story has appeal, particularly for fans left wanting by the first Suicide Squad movie, I personally suspect Phillips’ interests lie elsewhere. Further Scorsese has left a few blueprints in this direction as well.