The Joker is the most famous supervillain in the world. His popularity and ubiquity have netted him a spot in pop culture canon that will never be stripped away, putting him firmly in the upper echelons with characters like Darth Vader and Jason Voorhees. You can distill him down to his most basic components: purple outfit, green hair, and giant, terrifying grin, and he'll still be recognized, even by a person who's never picked up a comic or sat down to watch a movie like The Dark Knight.
It's that status that made the announcement of a Joker solo movie pretty much inevitable. He's just too famous to not get the feature film treatment in this day and age--even though he really, really doesn't need it.
The thing that makes The Joker as popular as he is today is also the problem: He's a prism more than he is a character. His single most defining trait is his ability to transform into whatever Batman isn't, based on whatever the story may need--a nice little skill that makes him endlessly and impossibly elastic for both the needs of the narrative and, perhaps more importantly, the needs of his fans. No one's interpretation of The Joker is ever wrong, because no one's interpretation of The Joker is ever completely right.
That elasticity means that, as far as supporting characters are concerned, he's the total package. No matter how specialized and expansive Batman's rogues gallery becomes, the Joker is always going to be a step ahead of the rest just by virtue of his ability to fill any gaps in any lineup. But isolated by a spotlight? Things start to crumble fast. A prism without anything to refract is just another chunk of glass.
In a solo context, is the Joker a murderer with a fixation on clowns? Is he just another petty thief in Gotham City? A mob boss? An asylum escapee turned serial killer? A sociopathic but ultimately comical ne'er-do-well? Who knows? He's certainly been all of those things and then some across the last 60-some-odd years. And what about his origin story? Is he a two bit crook who suffered a chemical accident in a run in with Batman? A regular guy who snapped? A cosmic wraith who represents the greater force of entropy in the universe? No one can really say for sure. In many ways, he's all of those things--he specifically needs to be all of those things, because that's what keeps his clown car running.
The Joker isn't a character whose presence in a story demands answers to these questions. That's not what keeps him going. Instead, he thrives on prompting more questions. That's what makes him such perfect support for other characters' stories--while Batman will always represent concrete black-and-white and the absolute idealism of his vigilante code, the Joker will always represent the lack thereof. But stripped of that context and put under a microscope all his own, he becomes shapeless and void.
Of course, there's also the matter of the cliches at hand. Fans love to critique Batman movies for their endless repetition, gunshots and pearls hitting the ground in Crime Alley, but next to the origin story, the looming thread of the Joker has never not been present. From Romero to Ledger, live action Jokers have smeared their grease paint stained hands across Batman media for decades, with Leto's tattooed and "modern" interpretation just the latest entry in the pantheon.
We may have yet to get a full movie with a Joker who wears silver teeth and has the word "damaged" literally stamped on his forehead, but we already know who he is, the same way we've always known who he is. We've reached the point where the endless permutations of this endlessly permutatable character have started to deliver diminishing returns. Unlike Batman, who sustains repetition by consistency, The Joker keeps momentum by having none--he can't be observed for too long in any one state or he starts to overstay his welcome and lose his mystique.
Even a 90-minute popcorn crunching romp would just be too long to risk; it would require too many answers be given to unanswerable rhetoricals, would ask for too many mysteries to be solved, and would see too much close inspection of a character whose heart beats in the margins and on the periphery. Like trying to look at a bug under a magnifying glass and accidentally lighting it on fire, a Joker solo film will likely go down in superhero movie history as gratuitous at best and a nail in the coffin of a struggling film franchise at worst.
There are certainly ways to sneak past the complications of a Joker oriented outing that don't explicitly put him in focus--source material inspiration like Joker by Brian Azzarello and Lee Bermejo that, essentially, show The Joker's presence in the city by way of his effect on bystanders. Ultimately pulling Batman into the mix might also be a good way to go. Peppering in elements from other Gotham-based stories like Gotham Central, which can zoom in on members of the GCPD, might also be a potential solution and a way to create a movie that Joker centric in name only.
Though maybe a nice catastrophic burial is the only way the Clown Prince of Crime will ever be allowed to rest away from movie theatres and reboots for the next couple decades--and in that case? Let's just get this over with.
