Nathan Fillion Is [Spoiler] In Guardians Of The Galaxy Vol. 2
If you are not sure who Simon Williams is, he use to be a member of the Masters of Evil. His Brother, Eric Williams, goes by the Grim Reaper. Later Simon was reformed and joined the Avengers. He would eventually retire from the team to start an acting career.
Going to guess that the posters are just meant to be easter eggs, so we probably wont be seeing Nathan as Wonder Man fighting next to Star Lord in Guardians 2