Well he is John Diggle Stewart because his military step-father is Roy Stewart. On Earth-Prime, it would be great if Roy Stewart was Lynn Stewart's father. They were dropping hints that John could be the GL well before Elseworlds. So it's not like fans were reacting to nothing.
Since there's plans for an HBO Max Green Lantern series, it seems unlikely that David Ramsey would get to be a GL in the Arrowverse. But since he and Lyla are moving to Metropolis, if they were to appear on the new CW Superman show, I think there's a better chance that John could be a GL on there--for two reasons, one being that DC's big two seem to have a lot more pull for their shows and the other being that a Superman series is likely to get a much bigger budget for effects. I can see WB giving Superman everything in the cookie jar for his series--including Green Lanterns and New Gods.