The clue is in the title here: if the regular Justice League is the cornbread, PG-13 version of the superhero supergroup, Justice League Dark can be its weirder, gnarlier R-rated equivalent.
With John Constantine leading a gang of supernatural weirdos, JLD has a distinctly different flavour to the core Justice League, and if Warner Bros. took their time to find the right cast and director, this could effectively end up a more adult-orientated spin on the MCU's Guardians of the Galaxy.
By introducing full-tilt horror elements, perhaps with some grisly gore, JLD would bring some appropriate grimness to the DCEU, rather than trying to foist R-rated elements onto characters that it doesn't suit.
How Likely Is It?: Guillermo del Toro was trying to get the movie made a few years ago, and though Doug Liman was recently attached, he's since departed, leaving the project without a director.
Considering the clear cost that would be involved in bringing JLD to the big screen, there's very little chance of it getting an R-rating, sadly. If it even happens, it's most certainly going to be PG-13. 3/10