Sloppy casting call announcements have revealed key plot points and characters before, so it's easy to believe that may have just happened again with the two-part The Avengers: Infinity War.
Auditions are now getting underway for the Marvel Studios feature film Avengers: Infinity War – Part I lead and supporting roles. The auditions for new lead and supporting roles will be taking place in Los Angeles, CA. The film is the first in a two-part sequel in which Thanos plays a major role, along with the Magus. Adam Warlock’s bad side, the Magus, separates from his body and assembles the Infinity Gauntlet. Magus creates doppelgangers of Earth’s heroes as part of his plan to recreate a universe of evil.
Does that mean key Guardians of the Galaxy comics character Adam Warlock and his villainous alter ego Magus are going to show up? Sure seems like it, but obviously its only a rumor at this point.
That's not the only thing revealed by this alleged casting announcement. It goes on to list the main actors and their characters in the film. That list includes Tom Hiddleston as Loki, who has also been rumored for a while, but this could (almost) confirm it.