Warning: Spoilers for Arrow Season 3 finale, if you are not caught up do not read!
As many of you may know, at the end of the Arrow season 3 finale Oliver told Diggle and I quote, "If you're going to keep fighting, you should find a way to hide your face out there." Now this is obviously alluding to Diggle being a hero, but who? John Diggle was created for the TV series, so therefore he exists exclusively in the "Arrowverse," and there is no comic origin for his character to be based on. I have three possible theories on who Diggle could become, and I hope at least one of them sheds some light on the character.
Green Lantern (John Stewart)
I bet you're tired of hearing this theory, but mine has a little more of a spin on it. In season 3 of Arrow and season 1 of The Flash, they've made a couple of references to the possibility of Green Lantern existing. In The Flash episode Rouge Air, Barry makes a reference at the closed down airport, saying "one of their test pilots went missing." This is a reference to Hal Jordan, and they have also mentioned Coast City in Arrow at least once. References to Hal Jordan doesn't necessarily mean that Diggle is Green Lantern, but if Hal Jordan exists, then it's possible. For one, his name is John. His last name is different, but my theory on this is that they will reveal his middle name to be Stewart, like they did at the end of The Dark Knight Rises with Joseph Gordon-Levitt.
Another thing is, Hawkgirl is going to be introduced in Legends of Tomorrow, and as you may know John Stewart and Hawkgirl have a romantic relationship, and actually have a child who becomes Warhawk. In the series, Diggle is married to Lyla Michaels aka Harbinger.
This could work in favor of the theory, since Harbinger dies in the comics when her and Supergirl are on Themyscira during Crisis on Infinite Earths.
This could cause a tie-in with the upcoming Supergirl series, and start Crisis on Infinite Earths in the Arrowverse. Harbinger was also a member of the Black Lantern Corps, which could mean that after her death, she goes after John out of jealousy.
That's right, Green Arrow, not the Arrow. If you read in the picture above, Green Arrow is mentioned for helping take down the Reverse-Flash. In the Arrow season 3 finale, Oliver quit being the Arrow, and drove off with Felicity into the sunset. We've seen Diggle put on the suit before, and criminals actually believed he was the real Arrow, so he obviously has the skill to be Green Arrow, so what if he makes his own suit to resemble Oliver's? This would stop him from getting mixed up with the former Arrow, and be seen as a different crime fighting entity.
A new hero
Diggle was created for TV, so it would make sense that they would create a new hero for Diggle's character. It sounds like the kind of unique idea that DC would come up with, and none of the fans would care because there is no origin story behind the character. No matter what the writers of Arrow decide to do, I have faith that they'll make Diggle's hero great.
What do you think? Are these theories likely? Let me know in the comments!