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Taskmaster (Tony Masters) — Nathan Fillion
Skilled at replicating any physical ability (like combat skills), Taskmaster has walked the line between being a hero and a villain. A fan-favorite character in the Marvel universe, Taskmaster also trains henchmen for evil organizations, which could be how he factors into the world of S.H.I.E.L.D. Since Nathan Fillion is already on ABC in Castle — and starred on Joss Whedon's Firefly — it's plausible he could play the masked mercenary.
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Spider-Woman (Jessica Drew) — Eliza Dushku
Joss Whedon is well-known for writing strong female characters, which makes Jessica Drew an ideal character to introduce — though likely without the Spidey costume. Captain America already introduced Hydra, with whom Spider-Woman has ties before becoming a S.H.I.E.L.D. agent, which would make for a great recurring nemesis for Coulson's team. Who better to play an enemy-turned-ally than Buffy and Dollhouse alum Eliza Dushku?
3 of 10 Marvel Comics; Isaac Brekken/WireImage
Power Man (Luke Cage) — Michael Jai White
Though there was speculation that J. August Richards would be playing the super-strong criminal-turned-hero for hire in the S.H.I.E.L.D. pilot, Marvel bosses have gone on record saying he's portraying a different character. The good news? Luke Cage could still appear at some point! Michael Jai White will head to Arrow this season as Bronze Tiger, but that shouldn't preclude him from taking on this tough guy role.
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Iron Fist (Danny Rand) — Ray Park
How can you introduce Luke Cage without his partner Danny Rand? The duo could plausibly form Heroes for Hire, which Coulson's team could investigate on the series. Plus, since that other billionaire philanthropist is unlikely to appear on the show, Danny could fill that role. Heroes and Star Wars alum Ray Park certainly has the skills required to emulate the martial arts master.
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Quake (Daisy Johnson) — Summer Glau
What's a Joss Whedon series without Summer Glau? The Firefly and Dollhouse alum could play the S.H.I.E.L.D. agent with the ability to create seismic waves, aka earthquakes.
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Jewel (Jessica Jones) — Abigail Spencer
ABC already toyed with the idea of doing a series featuring the former super-strong superhero-turned-private investigator — aka Jessica Jones — so it's not implausible that the character appear on S.H.I.E.L.D. as either a friend to the team or one of their potential targets to investigate. Rectify star Abigail Spencer would be great in the role as the everywoman superhero who goes on to marry Luke Cage.
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Scorpio (Jake Fury) — Omar Epps
If the series hasn't planned on including Nick Fury, introducing his younger brother Jake would be a good tie-in to the films and could potentially pave the way for a Samuel L. Jackson cameo. Coulson's team could investigate the Zodiac key, which Jake possesses as Scorpio while working with the Zodiac criminal cartel, which has great potential to also be a recurring nemesis for S.H.I.E.L.D.
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Mockingbird (Barbara "Bobbi" Morse Barton) — Laura Vandervoort
The martial artist and gymnast is not only skilled in hand-to-hand combat, but is also an adept scientist — which could cause some fun jealousy with the team's resident go-to gal Jemma Simmons. She will also eventually marry Hawkeye, which could facilitate a through-line to The Avengers without them actually having to appear. Laura Vandervoort has already shown she can play a superhero after her stint as Supergirl on Smallville, making her an ideal candidate for the blonde bombshell.
9 of 10 Marvel Comics; Paul A. Hebert/WireImage
Agent 13 (Sharon Carter) — Emily VanCamp
The Revenge star was rumored to be playing Sharon Carter, the niece of Captain America's love interest Peggy Carter, in the upcoming Captain America sequel — which was pretty much confirmed in a press release calling her Agent 13. A love interest of the Cap, the markswoman and martial artist could bridge the gap between the upcoming Phase 2 films and the show, an easy feat for VanCamp, who already stars on ABC's Revenge.
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Clay Quartermain — Ryan McPartlin
A high-ranking and fast-talking S.H.I.E.L.D. agent, Quartermain is a tougher character to introduce since he heads up S.H.I.E.L.D.'s Paranormal Containment Unit — the show isn't big on powers that can't be scientifically explained. However, he does have connections to Jessica Jones as well as Thunderbolt Ross, the future father-in-law of Bruce Banner. It would be awesome to see the ex-Chuck star pop up to lend Coulson's team a hand.