It appears as if Avengers: Age of Ultron will have a second villain beyond just the title villain. Latino Review, who has a pretty strong track record in regards to Marvel Studios scoop, is reporting that Marvel is looking to cast the role of Baron Von Strucker. The role is described as a supporting role, so Ultron would still be the main villain.
The role is described in Marvel's casting breakdown as a physically imposing Caucasian man in the forty to fifty age range. In the comic books, Baron Von Strucker is a supervillain and one of the leaders of HYDRA. He is nearly ageless, and he has a propensity to return from the dead.
Quicksilver & Scarlet Witch will reportedly work for Baron Von Strucker, which makes sense because director Joss Whedon has been a little mysterious when it comes to the role that Quicksilver & Scarlett Witch will play in the movie. He's never specifically stated they will be members of the Avengers in the film.
Martin Csokas and Dougray Scott are reportedly two actors that Marvel Studios is looking at for the role.
Avengers: Age Of Ultron is scheduled to be released in movie theaters on May 1, 2015.