[Potential MILD SPOILER for Guardians of the Galaxy Vol. 2 ahead.]
It's no secret that Guardians of the Galaxy Vol. 2 director James Gunn and Firefly and Castle star Nathan Fillion are friends. Fillion received a heavily-costumed cameo in the original Guardians of the Galaxy, though it wasn't exactly obvious which alien he was if you didn't already know.
Now, it seems that Fillion may have a more obvious cameo as an easter egg for eagle-eyed fans in the Guardians of the Galaxy sequel. He's been spotted on fake movie posters where he's the face of Simon Williams, aka Wonder Man from the comics.
Comic Book reported that Fillion's likeness appeared on several mock movie posters at a filming location in Cartersville, GA. The posters were advertising a "Simon Williams film festival" and showed Fillion-as-Williams playing in a romantic comedy called Oh Rebecca, a fantasy film called Arkon based on the Conan-like Marvel character of the same name, an apparent modern-day sequel to the 1922 silent film Haxan: Witchcraft Through the Ages and a Steve Jobs-esque biopic titled Tony Stark. Some were designed in a purposefully dated style, suggesting either that they would appear in the background of a flashback scene or that Williams has been acting for quite a while (with the Stark biopic possibly suggesting that it's the latter).
In the comics, Williams was originally a villain alongside his brother Grim Reaper in the Masters of Evil. The son of a rich industrialist, his family's munitions business starts losing money due to competition from Tony Stark. Blaming Stark for his woes, Williams is given powers by Baron Zemo and sent to infiltrate the Avengers and betray them at a key moment. Wonder Man eventually turns on Zemo and rescues the Avengers, seemingly at the cost of his own life, though he eventually returns and becomes a full-time Avenger and was a founding member of the West Coast Avengers. He eventually becomes a stuntman and well-known actor, usually wearing sunglasses to hide the fact that his eyes glow red as a result of his super-powers.
It's certainly too much to hope for to see fake trailers for some of these Simon Williams films, even though it would be an amazing bit of marketing. If nothing else, a one-shot trailer for Tony Stark would slay fans, especially if we could get some sort of reaction from Robert Downey Jr's Stark. Unless Marvel decides to make it happen, we'll just have to be content imaging Stark complaining that Williams looks nothing like him while wondering aloud who authorized the film in the first place.
Most likely, Fillion himself won't appear in Guardians of the Galaxy Vol. 2 since it would be odd for a high-profile actor like Simon Williams to attend what looks like a small-town film festival. It's also unknown whether any of his comic backstory would transfer over to the MCU or if Simon Williams is just an actor there (as he was in the "House of M" storyline in the comics). It's entirely possible that this is just another one-off reference, though if there's more to it then we probably won't find out until the Avengers: Infinity War movies.
Captain America: Civil War opens in theaters May 6, 2016, followed by Doctor Strange – November 4, 2016; Guardians of the Galaxy Vol. 2 – May 5, 2017; Spider-Man: Homecoming – July 7, 2017; Thor: Ragnarok – November 3, 2017; Black Panther – February 16, 2018; Avengers: Infinity War Part 1 – May 4, 2018; Ant-Man and the Wasp – July 6, 2018; Captain Marvel – March 8, 2019; Avengers: Infinity War Part 2 – May 3, 2019; and as-yet untitled Marvel movies on July 12, 2019, and on May 1, July 10, and November 6 in 2020.
Source: Comic Book