Nathan Fillion's cameo in Guardians of the Galaxy Vol. 2 has been kept under wraps for quite some time, but thanks to a series of photos uploaded by a user on Imgur, it may have finally been revealed.
Many of the photos seen below point to Fillion as Simon Williams, also known as Wonder Man, with one of the photos featuring a movie theater that showcases Williams' name. In the comic book series, Williams eventually becomes Wonder Man, who later becomes a member of the Avengers. He gets his powers from Zemo and the Enchantress, allowing him to have superhuman strength and speed.
Williams becomes an integral part of the Avengers team, heading out West to start up the West Coast Avengers, but in his spare time, decides to pursue a career within Hollywood. Hence the use of the movie theater and the posters showing off his various roles. Fillion's role confirms that the Avengers are at least around in this universe, but it's still unclear as to whether the actor will actually show up in the movie or if this is the extent of his cameo.
Although there are future Avengers movies on the table (Infinity - Part 1 and Infinity - Part 2), Fillion's involvement in the movies is questionable. This could be a way of introducing his character to Marvel's Cinematic Universe, but as of right now, it's safe to assume this is a one-time deal for the actor.
Guardians of the Galaxy Vol. 2, which stars Chris Pratt, Zoe Saldana, Dave Bautista, Vin Diesel and Bradley Cooper, will be released on May 5, 2017.