Re: The Avengers 2! The Official News and Speculation Thread - Upgrade section 4
It's also possible someone gets a hold of Stark's tech and utilizes it to create Ultron in some way, but Stark being the creator is looking fairly likely. Either way, the Iron Man mask being the key focus in the teaser definitely telling us that he plays a very big role in the process, as well as Joss saying Ultron comes from "the current team."
I wonder how SW/Quicksilver will factor into this story.