Apparently when Tom Hiddleston flatly denied he would be in the Avengers sequel, Age of Ultron he was either mistaken or fibbing, either being totally possible since he’s a little too perfect to play with trickster god.
Regardless, what we can probably look forward to is a Loki’d stinger (the clip after the credits of the movie) to tease Thor: Ragnarok. Why? Because the news about Hiddleston’s involvement with Age of Ultron comes from Heimdall aka Idris Elba who confirmed filming a scene with Hiddleston and Chris Hemsworth aka Thor. Had Loki been a major character in Age of Ultron not only would that have been marketed (the Loki’d masses would come running) but BTS pictures would have been leaked by now. But a short scene after the credits? That’s totally possible.