As great as Constantine was, especially Matt Ryan's portrayal, the show never found a solid footing in the writing and failed to grasp a loyal enough fanbase to warrant a second season. However all is not lost, as Constantine, as played by Matt Ryan, will live again on the CW when he appears in the fifth episode of the fourth season titled "Haunted". The press release for this states the following:
John Constantine is a weather worn exorcist and expert on the supernatural and the occult. He is a detective, magician, and a hunter of demons, and will provide critical support to Oliver (Stephen Amell) when arrows aren’t enough.
With last year's inclusion of the Lazarus Pit, The Flash introducing multiple timelines and meta-humans, and the forth-coming Legends of Tomorrow expanding time travel and introducing the near-immortal Vandal Savage, it seems the CW is really expanding it's reach into the DC archives as well as more science fiction-like territory.
Here's hoping that does well, and leads to more guest appearances for John Constantine. Or maybe even a series renewal of his own...