Marvel is casting for a second villain for Avengers 2: Age of Ultron; they are currently searching for an actor to play Baron Von Strucker, according to the usually reliable Latino Review.
A former Nazi officer (and one of the leaders of HYDRA) Baron Von Strucker was created by Stan Lee and Jack Kirby, and first appeared in Sgt. Fury and his Howling Commandos #5 in 1964.
Von Strucker is apparently involved in bringing Quicksilver and the Scarlet Witch into the storyline, since they will work for him at some point in the movie.  Elizabeth Olsen has been cast as the Scarlet Witch (see "Cooper, Olsen in Talks for Marvel Movie Roles") and Aaron Taylor-Johnson will play Quicksilver (see "Possible Quicksilver for 'Avengers 2'").