According to the Hollywood Reporter, J.J. Abrams and his company Bad Robot is not only working on a Justice League: Dark show for HBOMax but also companion shows for John Constantine and Zatanna. The information about this was overlooked at first within the article about John Cena’s and James Gunn’s Peacemaker announcement.
…J.J. Abrams has pitched a tent in the Justice League Dark territory of the DC Universe, working on supernatural shows featuring Constantine and Zatanna, among others.hollywoodreporter.com
John Constantine has had a few outings in Film and television. Most recently Matt Ryan has portrayed Hellblazer within the Greg Berlanti Arrowverse and seen on the show Legends of Tomorrow. In 2005 Keanu Reeves portrayed the role of John Constantine in a now cult classic movie simply titled Constantine.
While details are still scarce, it has been speculated that J.J. Abrams has been given a sandbox filled with Justice League: Dark characters to which he is free to play with on HBOMax.