Constantine was cancelled by NBC after a single season, but fan outcry led to Matt Ryan's version of the character showing up in Arrow and later becoming a regular in Legends of Tomorrow.
Recently, we learned that J.J. Abrams is a developing a Justice League Dark series for the upcoming HBO Max streaming service, and it's likely that a completely different actor will be cast as John Constantine in that show. It's possible, though unlikely, it will be set in the Arrowverse, but whatever the case may be, Ryan is hoping he'll at least get an audition.
"I'm a theater actor, you know? That's where I spent most of my life," he said while promoting Justice League Dark: Apokolips War. "I always say to them, 'Just get me in the room.' And then if they don't want me for the role that's their choice. But I would be very disappointed if a project like that went ahead without even sitting down and seeing what I could do with the material."
There will inevitably be a vocal demand from fans to see Ryan in Justice League Dark, but it's believed that the DC TV shows on HBO Max will boast much bigger budgets than those on The CW, and that likely means bigger name actors will be attached to star.
However, if Ryan can continue playing Constantine on Legends of Tomorrow, that will be no bad thing.