It's known that Scarlet Witch and Quicksilver team with Ultron in The Avengers 2.
Empire Magazine has a featured article with Elizabeth Olsen and asks the actress how that comes about.
Obviously there are plenty of countries in the world that hate America, for whatever historical pain they believe it has caused their own country and their own lives. That is the world we have created for Wanda and Pietro, to have been brought up in. What you're told a lot of times growing up is that that country, those people, their way of living is the reason why we have been bombed. They [The Avengers] are the worst evil in Wanda and Pietro's mind, not this guy who hasn't done anything yet.
Olsen also talks more about Scarlet Witch's powers.
The extent of our powers isn't fully explored in Ultron. But she can manipulate objects. And she has visions, and she has the ability to share them.
In the Marvel Comics, Scarlet Witch gets romantically involved with the Vision. Will it happen in the movie?
[Laughs] I don't know what you're talking about...
"The Avengers: Age Of Ultron" hits May 1, 2015 directed by Joss Whedon starring Robert Downey Jr., Chris Evans, Chris Hemsworth, Scarlett Johansson, Mark Ruffalo, Jeremy Renner, Samuel L. Jackson, Elizabeth Olsen as Scarlet Witch, Aaron Taylor-Johnson as Quicksilver, Paul Bettany as Vision, with James Spader as Ultron.
When Tony Stark tries to jumpstart a dormant peacekeeping program, things go awry and Earth’s Mightiest Heroes, including Iron Man, Captain America, Thor, The Incredible Hulk, Black Widow and Hawkeye, are put to the ultimate test as the fate of the planet hangs in the balance. As the villainous Ultron emerges, it is up to The Avengers to stop him from enacting his terrible plans, and soon uneasy alliances and unexpected action pave the way for an epic and unique global adventure.
The Avengers 2 Trailer: