Diggle would make a great Green Lantern (or Green Arrow for that matter) but I think they are going for a Birds of Prey spin off with the Canaries (love seeing both Canaries and [New] Green Arrow in action together BTW).
But besides Diggle, I would rather see John Stewart as Green Lantern. According to his bio we both served around the same (?) time.
Whoever the ring slinger is, it will be great if he finally makes it to the (Spectre) Arrowverse. Some of my other wish fulfillments would be Kevin Conroy coming to Batwoman to be her Bat(wo)man Beyond mentor, seeing Lucas Fox become Batwing or Batman, Carrie Kelley as Batwoman’s Robin, and Kate Kane creating or bringing the Bat Family together, the aforementioned Birds of Prey/Canaries spin off; I could go on forever.
And my wish fulfillment isn’t too far off. Because I’ve wanted Black Lightning to interact with the rest of the Arrowverse since his season one debut, AND I’ve wanted to see him join the Justice League / Super Friends since he came out in 1970’s. (I know he was great in Young Justice, too.)