Joker explores the origins of the Clown Prince of Crime in a way we’ve never seen before. What’s more, it’s also something of an origins story for Batman, too, as young Bruce Wayne and Arthur Fleck’s lives intersect in ways that they, and the audience, don’t expect. So, it’s not hard to imagine other major figures from the Bat-mythos being out there somewhere in this version of Gotham City, going through their own origins story at the same time.
The movie does actually feature a nod to another Batman villain, but it’s not necessarily one fans would expect to get a mention. Early on in the runtime, a news broadcast on TV mentions how Gotham is being overrun with “Super Rats.” The threat of these dangerous rodents is undercut by a subsequent joke that “Super Cats” should be enough to take care of them, but comic book readers may note the importance of this reference.
Otis Flannegan AKA Ratcatcher was a, well, ratcatcher by profession who eventually turned to a life of crime, using his peculiar ability to communicate and control rats to do so. Under Ratcatcher’s influence, his rats appear to have increased strength, endurance and even intelligence, so they could be labelled as “Super Rats.” Is Flannegan at work in Gotham’s sewers then while Fleck is causing havoc above? It’s possible.
It’s curious that this little-used villain gets a nod in Joker as the character is actually set to appear in The Suicide Squad. However, writer/director James Gunn has gender-flipped the foe for his movie, with the part being played by Daniela Melchior. It’s probably just a coincidence, and the fact that Joker isn’t part of the DCEU means this isn’t directly setting up the villain, but it’s an interesting connection nonetheless.