Marvel Comics; Michael Tran/FilmMagic
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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.