Writer/director Joss Whedon has been in pre-production for several months on the ensemble movie sequel and is on at least script draft No. 3 of the third highest grossing film of all-time. Once the holiday season comes and goes, and the calendar is swapped out for a new one, principle photography and production is a full go on Avengers: Age of Ultron in London.
Actor James Spader has already been officially announced to provide the voice and what sounds like a motion capture performance for the film's title card villain. And in the meantime, fans are still waiting on some official casting announcements for new team additions Scarlet Witch and Quicksilver, most likely to be called Wanda and Peitro Maximoff, respectfully. (As of right now, Elizabeth Olsen and Aaron Taylor-Johnson are the front runners.)
Studio president Kevin Feige recently spoke to Slash Film about the next Avengers film and he implied that Ultron's origin is something the Avengers see coming.
"[T]he threat that Ultron brings is certainly worthy of all the Avengers needing to come together and also the timing does play a large part of that. When you see a threat coming you have more time to assemble. There’s not a whole lot more I can say on that."
The most important part of Feige's quote is the second-to-last sentence. Even though it can be considered as reading in between the line, it hints that Ultron's origin will be something the Avengers see coming and, therefore, they have time to prepare for battle. Or this is Feige doing what he does best and that is create misdirection with some of his comments.
Of course fans have pointed to Tony Stark and JARVIS as being part of the creator story for Ultron but it also sounds like more hints at Ultron's creation could be teased in Thor: The Dark World, Captain America: The Winter Solder and/or Guardians of the Galaxy. In other words, pay close attention to the Phase 2 films because some of them may have clues to who Ultron's creator is and how it happens.