Feature Film Casting has a habit of posting inaccurate information with these early casting calls, but every so often they do land a nugget, making it a bit difficult to completely dismiss the info below. Keep in mind, this is the site that claimed
Black Widow would be in Iron Man 3. Clearly, Scarlett Johansson made no appearance in Shane Black's film. By the way, the site is a legit casting site, but at times their film plots, provided with their casting notices seem pieced together with public information. Below is the synopsis that they posted with the casting call.
In "The Avengers: Age of Ultron" Loki is gone and the new villain will be the evil robot Ultron, who has the ability to keep upgrading his own strength, making him extremely fearsome. Tony Stark/Iron Man becomes the unwitting creator of Ultron. The brother-and-sister team of Quicksilver and the Scarlet Witch become the seventh and eighth members of The Avengers. Quicksilver has extreme speed, while Scarlet Witch can cast spells and has telekinesis ability. Black Widow and Hawkeye will be key figures in the film.
What we know so far, from director Joss Whedon, is that Hank Pym (Ant-Man), will not be in
The Avengers sequel, but his creation, Ultron, will be. So, how is Ultron gonna be created if Pym isn't there to do it? Well, the most likely scenario is that the brilliant Tony Stark will have a hand in it. Best guess, is that they will borrow pieces of the original Jarvis storyline from the comics. In that, Ultron Hypnotizes Edwin Jarvis, turning him against
The Avengers. Perhaps this is plan for the featured film, but we'll have to wait and see.