19. The Movie's Second Mystery Villain
Marvel always wanted to have Loki as the main villain of The Avengers, but Joss Whedon wasn't initially convinced that the God of Mischief would be a great enough threat to Earth's Mightiest Heroes. He actually wanted to add a second, more formidable bad guy, but when the studio nixed that idea he went for the Chitauri instead so that the team would have something to hit. The filmmaker has since refused to reveal the identity of this second villain because there were plans for him or her elsewhere in the Marvel Cinematic Universe, but you have to wonder if it could have been Thanos or Avengers: Age Of Ultron's lead baddie.