There isn't enough time to give every Marvel character their own origin story movie in the Marvel Cinematic Universe - so upcoming team-up sequel The Avengers: Age of Ultron is going to help out by introducing several major superheroes. Actually, "heroes" might be a bit of a stretch, since twins Pietro and Wanda Maximoff (A.K.A. Quicksilver and Scarlet Witch) fight on Team Ultron (at some point) - and are ready to cause some real problems for the Avengers.
Not much has yet been given away about Quicksilver and Scarlet Witch in the trailers for Avengers: Age of Ultron so far; even the new featurette (see above) focusing specifically on those characters is pretty evasive. It largely consists of writer/director Joss Whedon and the various actors explaining what the twins' powers and personality traits are - and there are only so many ways to say "Quicksilver is very fast" before it starts becoming repetitive.
Interestingly, Scarlet Witch is repeatedly described as the more "grounded" of the two siblings, despite the fact that Elizabeth Olsen has previously described her character as being a bit unstable. To be more specific, "She's crazy. She has more things wrong with her than any character I've ever played." It's possible that at the time she was referring to the comic book version of Scarlet Witch. Still, if that's the case then it could be interesting to see a version of the character who's a bit more in control.
Even if Wanda Maximoff is mostly sane, she still has the ability to drive other people crazy using her magical abilities. Clips so far have shown her getting inside the heads of several different members of the Avengers, apparently instigating a huge battle between the Hulk and Iron Man.
Scarlet Witch's powers are so debilitating, in fact, that Olsen explains the Avengers have to "go somewhere and come back as a unit," which sounds uneasily like their story arc in The Avengers. We probably don't need to see these superheroes figuring out how to work as a team all over again, especially with all the other plot elements that are going to be introduced in this film (including the set-up for upcoming movies like Black Panther).
One new bit of information that the featurette does offer is the detail that the twins were left orphaned by shelling that collapsed their building when they were children. If that shell had "Stark Industries" written on it, then it would certainly explain their animosity towards the Avengers.
The Avengers: Age of Ultron releases in theaters on May 1 2015, followed by Ant-Man on July 17 2015, Captain America: Civil War on May 6 2016, Doctor Strange on November 4 2016, Guardians of the Galaxy 2 on May 5 2017, Spider-Man on July 28, 2017, Thor: Ragnarok on November 3 2017, Avengers: Infinity War – Part 1 on May 4 2018, Black Panther on July 6 2018, Captain Marvel on November 2 2018, Avengers: Infinity War – Part 2 on May 3 2019 and Inhumans on July 12, 2019.