Basically you guys have seen young justice right? No? here is a clip of guardian!
basically you all have realized that many people think Diggle will become guardian in season 4 and here is where Constantine comes into the play. You all know this mask
That's Dr fates mask worn my Constantine in one of the last episodes of the first season know look at how Constantine's reflection in the mask. now look at guardians mask again.
So you realize that
A.) Constantine is coming to the CW universe E.g (Arrow,flash and legends of tomorrow)
B.) The mask Constantine has is identical to guardians mask.
And here is the leaked costume. the helmet looks identical except it is black. maybe he still will be guardian but a change of color. Honestly if Batman could be included in the cw i would think He is red hood but for now i think He's guardian.
Therefore i think Diggle will be becoming Guardian.