Although Guillermo Del Toro exited Warner Bros.' Justice League Dark film a few months ago, he's still shaping the course it will take. The screenplay he wrote, which includes DC's mystical characters like John Constantine, Swamp Thing, Deadman, Zatanna, and Etrigan The Demon, is still being used, and apparently one of his choices for a key role is as well.
That role would be Swamp Thing, and Latino Review says Del Toro wanted his buddy Ron Perlman to play him which everyone apparently agrees with. As for John Constantine, they report that Colin Farrell is the frontrunner while Ewan McGregor has been approached. Sorry, all of you hoping Matt Ryan from the failed TV series would get another shot. Meanwhile, Monica Bellucci is said to be up to play Madame Xanadu. There's also the possibility Cara Delevingne brings her Enchantress character over from Suicide Squad for a cameo, which would make sense.
If the plan really is to begin filming next year, which sounds kind of early considering all that Warner Bros. has on their DC Universe plate, then expect a director to be named and some concrete casting announced.