The list of directors rumored to be on the shortlist according to Joblo are Fede Alvarez of the Evil Dead remake, and Big Bad Wolves duo Aharon Keshales and Navot Papushado. The kicker here is that Mendelsohn won't make a final decision until he knows who his director will be, and it seems unlikely any of these choices are going to sway him. But y'never know. Considering Justice League Dark deals with DC's mystical and horror realm, Alvarez would seem to be the most likely option.
Del Toro's script, which includes characters like Swamp Thing, John Constantine, and Madame Xanadu, is still being used, but officially the film has yet to earn a start date.