You definitely don't want to read into that too much. These toy companies always throw in other characters not in the movie for their toy lines. For example, Hasbro's figures for Thor, Cap, Iron Man 2 all have a separate line with comic-based figures. Meaning they're labeled with the name of the movie but feature characters not in the movies.
Also, I couldn't imagine keeping this a secret. My money is on a completely CG character like Thanos.
And Hugo Weaving said after Cap that he was done doing the big blockbuster stuff for a while, if I remember correctly. Unless they recast Skull, which would suck.