From the nearby Coast City, the intergalactic police officer and member of the Justice League wouldn't necessarily need to have his powers to appear on Arrow. Instead, Justin Hartley, who played Green Arrow on Smallville, could appear as Oliver's ally Hal Jordan.
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William Tockman/Clock King
An enemy of Green Arrow, the Clock King is a perfect foil since he doesn't have any superhuman powers — he's a master planner who uses clock-themed gadgetry. It would be fun to see Doctor Who's David Tennant (he played a Time Lord after all!) as a super-villain who goes up against Ollie.
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Princess Diana of Themyscira/Wonder Woman
The Originals star Phoebe Tonkin's name came up early and often when The CW announced it was doing a Wonder Woman prequel called Amazon, so it would make sense for her to come on as the Amazonian warrior princess who dons the Lasso of Truth. She could even meet Oliver during his days on the island, though in the Arrow world, she'd probably just use a whip — but we'd think Ollie would like that.
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Sans the wings, Carter Hall could be introduced as a gruff museum curator who comes into conflict with Oliver. And who would be better than Almost Human's Karl Urban, who's proven his prowess for cynicism in the new Star Trek films?
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Jefferson Pierce/Black Lightning
Oliver could cross paths with the Olympic athlete after having a one-night stand with his niece Joanna Pierce, similar to how it went down in the comics. Almost Human's Michael Ealy would be a good partner for Oliver, of sorts, as they head towards the eventual Justice League.
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Barry Allen/The Flash
Having played Bart Allen/Impulse on Smallville, Kyle Gallner would be a great Flash on Arrow, hailing from nearby Central City. The series could ground him in reality by making him an everyday guy who, inspired by Starling City's vigilante, has the impulse to rush off and save people, even though he doesn't actually have any powers.
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As this femme fatale, Ringer's Zoey Deutch would make a good foil for Roy Harper, with whom she goes on to have a child in the comics. But we could see her stirring the pot now by getting between Thea and Roy and maybe even bringing out Roy's dark side.
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The introduction of Dick Grayson could be the connection to Gotham City we've all been waiting for. Though we'd hope for Joseph Gordon-Levitt, we won't hold our breath. Instead, White Collar's Matt Bomer totally has the body frame of the former acrobat who went on to work with Batman.
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The introduction of Nightwing could facilitate Kate Kane coming in as Batwoman, the openly gay vigilante who could be played by Amy Acker. Should she crossover into Starling City, Nightwing could come to town to try to get her back in Gotham to hold down the fort out there.
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Vic Sage/The Question
Castle's Nathan Fillion would undoubtedly be perfect as this master detective-turned-angry vigilante who follows the philosophy of Objectivism — and he would even make for a good love interest for The Huntress, similar to what happens in the comics. Better yet, Fillion could keep the mask on throughout the majority of the gig and only afteward, reveal his face, which would be a neat surprise for the fans.
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Sandra Wu-San/Lady Shiva
Once her tenure on Nikita is up, Maggie Q should definitely take on this kickass fighter who so rarely loses. She could tie into the introduction of The Question since she's his mentor in the comics, or she could be introduced as part of the Birds of Prey once Laurel becomes Black Canary.
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Dr. Harleen Frances Quinzel/Harley Quinn
Suburgatory's Jane Levy has the stature to take on Joker's girlfriend, the flamboyant adversary of Batman who could make her way to Starling City following The Glades being ravaged by the earthquake in the finale.
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The one character many DC Comics fans would love to see is also the one who will likely never appear on the series since the rights to Batman have always been trickier. But with Christopher Nolan's film trilogy finished, everyone can still hope to see the caped crusader appear, even sans the cape. Stephen Amell's own cousin, Tomorrow People's Robbie Amell would fit right in, especially since Stephen himself has already said he should do it.