Tom Hiddleston does return in Thor: The Dark World, debuting in theaters this fall, but it isn't known why the villain does not make an appearance in The Avengers 2. The filmmaker also confirmed a story from April that he has finished the first draft of the sequel, and that the rest of the Marvel's The Avengers cast is returning.
It isn't clear when production will begin at this time, but it will likely start sometime next year, after Guardians of the Galaxy finishes shooting.
Avengers: Age of Ultron comes to theaters May 1st, 2015 and stars Aaron Taylor-Johnson, Scarlett Johansson, Robert Downey Jr., Elizabeth Olsen, Chris Hemsworth, Chris Evans, Cobie Smulders, Jeremy Renner. The film is directed by Joss Whedon.