While this one might have seemed obvious considering the film is being called "Age Of Ultron" and not "Loki Returns Again," it's good to have confirmation. In an interview with Entertainment Weekly, Tom Hiddleston confirmed once and for all Loki's role in Avengers: Age Of Ultron.
Tom Hiddleston said, "He’s not in it. Here’s the thing: I don’t think there’s anything else Loki could contribute to The Avengers, narratively. Joss gave me so many wonderful things to do in that first film that we’d only be trying to repeat ourselves, and probably less well in the second one."
Tom Hiddleston added, "When I think of all the things I loved as a child, for example, Hans Gruber wasn’t in Die Hard 2. And in Indiana Jones, by the time he moved from the Lost Ark to the Temple of Doom, there was a whole new bad guy. I think keeping it fresh and new is good for The Avengers even though it’s a shame for me. And let’s face it, there’s seven or eight of them as it is, and Joss is thinking about adding two more, so the screen’s gonna be a busy place. But I had the time of my life on that film. It was the most fun ever.”
Hiddleston's confirmation of Loki's absence is sort of good news and bad news. Tom Hiddleston has been one of the bright, shining stars of the Marvel Cinematic Universe, so it's always a little bit of a disappointment when he's not in a film. However, in addition to being the main villain in The Avengers, Loki was also featured in the first Thor movie (and appears to be playing a major role in the Thor: The Dark World), so it really is time for a new villain to shine in the Marvel Cinematic Universe. Let the Age of Ultron begin.