While Ben Kingsley's appearance in Iron Man turned out to be controversial to say the least, the veteran actor has revealed that he is working on another secret project for Marvel Studios. In an interview with the Belfast Telegraph to promote Ender's Game, Kingsley said, "It's a secret Marvel project. I'm not allowed to say any more, you're going to have to wait and see."
Spoiler Warning: Of course in Iron Man 3, Ben Kingsley was revealed not to be the Mandarin, but instead to be an actor named Trevor Slattery hired to play the Mandarin. Interestingly enough, Kingsley refused to reveal if he would be reprising his part as the Mandarin. One of the big conspiracy theories that surfaced after Iron Man 3 was that there might be a double twist. Some fans speculated that Kingsley could really be the Mandarin, and the Trevor Slattery persona was just a cover that was all part of his master plan. Wouldn't it be a shocker if the Mandarin showed up as a group of supervillains in Avengers 3? It would be such a deliciously evil, unexpected twist that it might sooth the nerves of those still upset by Iron Man 3.
Another possibility is that Marvel Studios could be planning another of their popular short films. Maybe, something along the lines of a Life and Times of Trevor Slattery short. While such a short would likely be funny, it might not go over well with comic book fans, who were already upset by and would prefer to forget the Mandarin twist.
However, the most likely possibility is that Kingsley is working on a completely different project for Marvel Studios. While at first fans might question an actor playing two characters in the same Marvel Cinematic Universe, a lot of Marvel characters have rather unique looks and Kingsley is a master of disguise. He could play a character that is so visually different that general audiences wouldn't even associate him with his role from Iron Man 3. While this is pure speculation, Doctor Strange has been recently confirmed to be in active development, even though it hasn't been officially announced with a release date. There are a number of characters in the Doctor Strange universe, which would probably fit well with Kingsley's look and style of acting.
Marvel Studios President of Production Kevin Feige also confirmed that Ben Kingsley was working on a secret project, but he refused to give details.