The Wrap is reporting that the lead villain for the Birds of Prey (since we already know the leads of the film) movie will be-- Black Mask. Now, "one of Batman's deadliest foes" doesn't really describe the character. Hugo Strange. Bane. Joker. Riddler. Even KGBeast are on par with Batman's "deadliest" enemies. Not Black Mask.
The character was basically a mort villain until he became a "Red Skull" rip-off in Ed Brubaker/Darwyn Cooke's Catwoman run where his quirk of brutally torturing his victims. This little-added trait to the character came back up in the infamous Batman story/event, "War Games":
It's more than likely we'll be seeing this side to the villain. Likewise, his need not much to kill Batman, but become the crime boss in Gotham. Something that was used in "Under the Hood" and the animated adaptation:
Currently, Mask in Rebirth hasn't really displayed this streak and has gone a more S&M gear look when Scott Snyder and Scott Lobdell used him in their various Batman comics. But it stands to reason given the prior versions being the most well known that's probably the version of Mask that'll be represented on film.
Of course, given this is Black Mask. It sounds very much likely he won't be the only villain. Given if they go with the crime lord approach he'll probably not be the only villain to pester the heroes. Birds of Prey will begin filming in early 2019 for a 2020 release.