WARNING: The following article contains spoilers for Batman #90, by James Tynion IV, Jorge Jimenez, Tomeu Morey and Clayton Cowles, on sale now.
In Batman #90, the Designer's intricate plans for Gotham City were finally revealed. After sending assassins and zombies after four of Batman's most famous villains, the Designer returned to the city to reshape it in his own image. The specific details behind the deals that Catwoman and those other villains came to light. While the masked mastermind originally tried to make deals with Catwoman, the Joker, Penguin and the Riddler, he turned against the villains when he began to understand the Joker's madness. Although he was originally killed in battle with the Joker, the details of his death were apparently exaggerated.
Despite all that, the Designer's identity is still largely a mystery. However, he's mentioned that he defeated the world's greatest detective in the past. Although it may seem unlikely at first glance, this could mean that the Designer could be Moriarty, the nemesis of Sir Arthur Conan Doyle's iconic detective Sherlock Holmes. While that might seem implausible at a glance, it wouldn't be unprecedented for the Dark Knight.
Given what the Designer says about defeating the world's greatest detective in this issue. There's a very real chance that he could be the original Professor Moriarty, one of the most ingenious criminal masterminds in all of fiction. His rival, Sherlock Holmes, has been established as a fairly firm part of the DC Universe for some time. On television and in pre-Crisis on Infinite Earths comics, Holmes even teamed up with Batman on a few rare occasions. While Homes and Moriarty's battles are firmly set in the past, the DC Universe if full of ways to artificially extend one's life.
Even if the Designer isn't the original Moriarty, he could be a version of another Moriarty who already has a history with Batman.
In a story in 1946's Detective Comics #110, by Don Cameron and Win Mortimer, Batman encountered a new villain named Professor Moriarty. He was a Batman villain who shared a last name with Sherlock Holmes' famous archenemy. Since he effectively evaded Scotland Yard for so long, Batman and Robin were eventually called in to help track him down.
Like all of Batman's early adventures on the world that came to be known as Earth-2, this particular story was erased by Crisis on Infinite Earths. While he hasn't been since then, the Designer could be a new version of this obscure classic Batman villain.
Whether or not the Designer is some version of Moriarty, he seems to have his sights set on both Batman and Bruce Wayne. Even if a lot of his history isn't clear, the Designer has some remarkably well-considered plans, so he might have a little bit of Moriarty in him after all.