In the series finale of Arrow, Diggle stumbled across a box with a glowing green object inside that was obviously meant to be a Green Lantern Corps ring. The implication was that Oliver Queen's best friend was, in fact, a version of John Stewart, and the character is now set to return in upcoming episodes of Batwoman, Superman & Lois, and Supergirl.
The actor will also step behind the camera to direct, but will we get to witness Diggle's transformation into Green Lantern? Unfortunately (and unsurprisingly), that won't be happening.
"To your other point, which is exploring what has happened to Diggle since encountering that green box, that will be explored in the other episodes - on Batwoman, Flash, Superman & Lois, and Supergirl," Ramsey explained in an interview with TV Line. "Whatever was in the box, he refused the invitation of. Let’s put it like that. But there are consequences to that."
"After Crisis, he got back his wife, who was abducted by the Monitor, and he got back his daughter Sara, who was taken out of existence by Flashpoint. So part of Diggle’s story is that the last thing he would want to do right now is receive an invitation from something otherworldly, because he finally has his family back, and he lost his best friend to some otherworldliness," he continued. "So he refused the invitation of whatever was in the box, but there are consequences to that, and that story, what he will do next, is part of what we tell throughout these four episodes."
With Warner Bros. developing a Green Lantern Corps movie and a Green Lantern TV series for HBO Max, it was probably inevitable that The CWVerse wouldn't be allowed to introduce their own version.
The site even asked specifically whether any of the heroes Diggle crosses paths with will directly address the existence of an intergalactic group of space cops. "No, we don’t get into that," Ramsey confessed. "This is a very, very, very preliminary [look at] what happens from this refusal, and what it means to his destiny. We simply get into the preliminary physical effects of his refusal of whatever it was in that box."
Honestly, this all sounds a tad underwhelming, and probably not what fans of the character were hoping for. It's possible Diggle will one day "suit up," but for now, that's clearly not the plan for him.