AVENGERS: AGE OF ULTRON was already pretty jam packed as it was, but the film originally included just one other character, that of Tom Hiddleston's Loki. Hiddleston was set to reprise his character in Thor's dream sequence, however we learned fairly early on that Joss Whedon had cut Loki out of the film. At the time all Whedon would say about this decision was that despite really wanting Loki to be a part of it, he feared that the film would become "overstuffed." Now Tom Hiddleston has explained what really happened in an interview with Digital Spy:
I was part of the dream sequence for the character of Thor. I shot for a day and enjoyed it very much, and then I received a phone call from Kevin Feige [who] said that in test screenings, audiences had overemphasized Loki’s role. So they thought that because I was in it, I was controlling Ultron, and it was actually imbalancing people’s expectations so Joss and Kevin were like, "Let’s cut it because it’s confusing people."
That's a testament to how much people must love Loki, what probably amounted to little more than a cameo had everyone believing that he was the true brains behind the entire film. As fun as it would have been to have seen Loki once more, the decision was probably the correct one. Tom Hiddleston will return in THOR: RAGNAROK, which may have just landed a director, on November 3, 2017.