The "Elseworlds" crossover had so many massive reveals that a few things that came out may have been lost in the mix, including things like John Wesley Shipp's Earth-90 Flash seemingly mistaking John Diggle for a Green Lantern. Apparently, that could be a thing, but Arrow showrunner Beth Schwartz said John's Green Lantern won't be appearing anytime soon.
To be continued. No, we were all really excited about that fun nod. But I'm not sure what's to come of that, really.
Beth Schwartz continued to express that any episodes that may include a Green Lantern Diggle haven't been written yet. Comicbook.com reported The Flash showrunner Todd Helbing said the same, which may rule out the possibility that The Flash or Legends of Tomorrow is currently planning on introducing Green Lantern Diggle in an effort to keep Arrow grounded and powerless. For now, it appears the "Elseworlds" crossover's cool reference is merely an Easter egg at the moment.
That said, it's a very cool Easter egg that serves as a nod to long-held fan theories regarding John Diggle. Speculation has run rampant over the years that the original Arrow character would eventually become a Green Lantern. "Elseworlds" introduced a scenario where that could happen, if not through Earth-1's Diggle but through another Earth's iteration. Considering the state Earth-90 was in when its Flash escaped, it's unknown what became of that Earth's Green Lantern.
That said, it's hard to believe that the Arrow-verse would acknowledge such a long-standing fan theory and have no plans for it. While Beth Schwartz and Todd Helbing have said nothing has been written, it's possible there could be some tentative plan to include a John Diggle Green Lantern in some way. Given past remarks about the importance of establishing John Diggle's story as an original character, it may not be his Earth-1 counterpart that dons a ring.
Stranger things have happened, of course, and it wasn't that long ago that the Arrow-verse's cast and crew were being cryptic and non-committal about bringing "Crisis On Infinite Earths" to the franchise. With so many heroes planned to play a part in that grand adventure, this seems like the perfect opportunity to at least feature a scene or two of David Ramsay's John as Green Lantern.
Of course, with DC working on a Green Lantern Corps film, this could be a case of where it's not eager to have a Green Lantern appear in the Arrow-verse. Such things have happened before, and despite a Harley Quinn tease in Arrow's early seasons, the villainess has yet to appear in front of the camera in any non-deleted scenes. The same situation could easily happen again with this Green Lantern Easter egg, and showrunners are just getting out ahead of things before rumors spiral out of control.