9. Thanos
Regardless of how important the Infinity Stones end up being to the plot of the sequel, Thanos making some sort of surprise appearance is pretty much a guaranteed Easter Egg at this point. Chances are it will be in a mid or after-credits scene as fans are shown another glimpse of his plan to invade Earth and retrieve what he needs to wield the Infinity Gauntlet, but the first trailer might have inadvertently revealed how he will factor into proceedings. Captain America's shattered shield and the rest of the fallen Avengers on what looks like an asteroid is apparently a vision of Tony Stark's brought on by the Scarlet Witch's mind control powers. Iron Man obviously knows nothing about Thanos and has never met or seen the Mad Titan, but it's not hard to imagine Joss Whedon throwing some sort of appearance in for the villain here, even if he's just cloaked in shadow or merely hinted at somehow.