I didn't realize they were actually that serious about bring him over. I had always assumed that Amell talking about Constatine was more just joking around or wishful thinking.
I never doubted it was sincere. Berlanti is clearly a huge DC fan and loves building an interconnected DC universe on TV, so it's perfectly in character for him to want to connect to Constantine.
Hey, imagine if John Constantine becomes the DC equivalent of Detective Munch -- bouncing around from show to show for years to come.
