Long before Avengers: Age of Ultron opened in theaters Idris Elba (Heimdall in the Thor movies) spoiled a little surprise from Joss Whedon's script, revealing that both he and Tom Hiddleston's Loki would be making cameo appearances. Whedon, alongside Hiddleston, both stated in interviews that Loki wasn't actually going to be in the movie but when it released Whedon confirmed that they did shoot something but it didn't quite fit the story and was therefore cut.
Some chalked it up to normal edits to the movie made to trim the running time, others to the widely publicized creative differences between Whedon and Marvel Studios' top brass, but it was actually a moviegoing audience who decided that Loki wouldn't be seen again on the big screen until Phase 3 of the Marvel Cinematic Universe.
While promoting Crimson Peak, Hiddleston was asked about his part in Avengers: Age of Ultron that never came to be and he told Digital Spy that the bit part he shot during reshoots was for Thor's (Chris Hemsworth) dream sequence induced by Scarlet Witch (Elizabeth Olsen). While bringing back Marvel's fan-fave villain would have been beneficial for marketing purposes and to tease Thor: Ragnarok it may have distracted from the the main story.
"In test screenings audiences had sort of over-emphasized Loki's role so they thought that because I was in it I was controlling Ultron and that it was actually imbalancing people's expectations so Joss [Whedon] and Kevin [Feige] were like 'let's cut it' because it's confusing people."
Hiddleston continued saying "it made sense to me" when he saw the film. Unfortunately, the Thor subplot didn't make sense overall. As the most knowledgeable and longest living Avenger, Thor was the first to pickup on the issue of the Infinity Stones rapidly surfacing across the galaxy and seeks out a ... pool of water to have a dangerous vision bath in. Most of these scenes were cut alongside elements of Thor's dream sequence so it felt out of place and borderline nonsensical. You can view the full Norn pool sequence here since it was released as a deleted scene on the Avengers 2 Blu-ray.
Hopefully this will be expanded upon in Thor: Ragnarok which Marvel Studios boss Kevin Feige explains will be very important to the MCU and setting up Avengers: Infinity War. We suspect Loki will have a very big role to play in both.
Captain America: Civil War opens in theaters May 6, 2016; Doctor Strange – November 4, 2016; Guardians of the Galaxy Vol. 2 – May 5, 2017; Spider-Man– July 28, 2017; Thor: Ragnarok – November 3, 2017; Avengers: Infinity War Part 1 – May 4, 2018; Black Panther– July 6, 2018; Captain Marvel – November 2, 2018; Avengers: Infinity War Part 2 – May 3, 2019; Inhumans – July 12, 2019.
Source: Digital Spy