Haven't seen "Joker" yet, but the trailers make me think it's a pity this is being pitched as a stand-alone one-off movie. It would be fascinating to see the same team try to make Batman work with this bleak, ultra-gritty vision.
Presumably they'd go with the bleakest, grittiest take on Batman - that he's every bit as much of a psychopath as Joker, and if the story was really rough around the edges, Bats wouldn't be as superhumanly effective a vigilante as usually portrayed. He'd take some big lumps.
Set the movie in roughly the same time period as "Joker" and Batman's advanced tech would be less advanced, giving him less of an edge. No magic bulletproof armor, self-driving vehicles, swarms of bats summoned on demand, etc.
Batman could still be formidable, especially if he's a really committed psychopath determined to lay cunning traps for his prey and really lean into using fear as a weapon, the way "Batman Begins" implied - but no tech or ninja magic.
Just as "Joker" looks like it's trying to work up a bit of sympathy for the devil, a Batman follow-up would methodically strip the audience of sympathy for the hero at first. He'd look like a dangerous lunatic. He would come close to getting himself and others killed.
His obsession with crime would clearly be just that, an obsession. The audience would be asked to really ponder how unhealthy it is to be utterly focused on the long-ago crime of his parents' murder, unable to move on with his life, and how his wealth weaponizes his madness.
But this Batman wouldn't be a hapless joke. He really is dangerous - physically powerful after training with maniacal focus, intelligent and cunning, fearless in his mania, cruel in his loathing for predatory criminals. Even with 70s/80s tech, he can have believably useful tools.
And a funny thing happens as the story unfolds: he gets better at what he does. He's stronger after his wounds heal. He loses his last few illusions and becomes street-smart. His reputation builds and the fear he inspires causes his prey to make panicked mistakes.
And then the people of this grimy, forlorn, desperate Gotham begin asking themselves: maybe we're better off with this particular psychopath running around. Maybe it's good to have an apex predator who feeds on other predators. Maybe it takes a maniac to fix this town.
You could ask some very challenging questions of the audience and turn Batman into a character every bit as disturbing as Joker, but in a different way. You could make a sequel that fits tonally with this one and makes some equally squirmy points about society and its outcasts.
The really neat trick would be ending up with a Batman that feels valid - it's the same story we all know, just told in a completely different way that asks if a damaged man whose psychosis benefits society is really a hero, or if we would make him into one. /end
