I can see so clearly how you could adapt John Diggle from CW’s Arrow into being John Stewart the Green Lantern.
They’ve already got the first name in common, and the switch from Diggle to Stewart could be accomplished by saying one was his middle name, or his regular identity was compromised, so he switched to his mother’s maiden name. That part’s relatively simple.
Diggle’s background is still in-line with continuity for Stewart’s history. He’s got the military experience, the by-the-books but takes-no-crap attitude, and it even sets up a pre-existing familiarity with crime-fighting, and the concept of costumed heroes and villains.
So John Diggle becomes John Stewart, who becomes Green Lantern.
While Arrow is between seasons, David Ramsey is in the JLA movie, which gives a tie between the TV series and the movie Universe. This is a profitable business move, and is hugely satisfying for the fans.
Suddenly, you’ve got an awesome Green Lantern, and he has an established history with Green Arrow. These guys are buddies who’ve been through years of adventures together, and now they’re two of the world’s most famous super-heroes.
After the JLA movie, David goes back to Arrow, and the Green Lantern Ring goes into a box that says: in case of emergency only. And we’d learn that, maybe John isn’t totally comfortable with using the ring.
“This thing… it’s like an atomic bomb. And I can feel it, Oliver. I can feel it in my head, when I wear it. Like it’s talking to me.”
The ring is an alien device. It’s eerie, otherworldly. Almost like the monolith from 2001. And John would respect the power of the ring too much to abuse it. “This thing isn’t for fighting crime, it’s for saving the world. Maybe other worlds, too.”
Perhaps once a season, John would use the ring, and something crazy would happen, and there’d always be some negative fallout. That way, when the ring came off he could say, “and that’s why I can’t use the this thing! Because somebody always gets hurt!”
But then, JLA 2 comes around… And he’s called to duty. The ring returns.
Or maybe, at the end of a season of Arrow, John admits that he needs more training, and he goes off-Earth to the Green Lantern Corps homeworld. That would parallel Ollie’s adventures and training on the island, and it could also lead nicely into an off-world Green Lantern movie. A space adventure, out in the stars.
Of course, this is all just me daydreaming about potential storylines, but who knows? Sometimes, really awesome things happen.