IGN spoke with Whedon, and brings us the following:
Whedon had previously alluded to a brother/sister duo being in the film, which EW later said was indeed Quicksilver and Scarlet Witch. When I brought up Quicksilver and the Scarlet Witch to Whedon, he confirmed they were the characters in question, explaining, “You know, they had a rough beginning. They’re interesting to me because they sort of represent the part of the world that wouldn’t necessarily agree with The Avengers. So they’re not there to make things easier. I’m not putting any characters in the movie that will make things easier.”
[Editor’s Note: Big spoilers for the end of Iron Man 3 follow.]
We also know Tony Stark/Iron Man plays another large role in the sequel, but the end of Iron Man 3 saw Tony destroying all the Iron Man suits we knew about, while seemingly stepping back from his time in the armor.
When I asked Whedon if that made it a challenge putting Iron Man in Avengers 2, he remarked, “Well, I feel like in Iron Man 3, even though he said, ‘I’ve changed' -- he blew up his remote suits, but I don’t think anybody thinks he doesn’t have one anymore. The question is, if The Avengers are called, does he show up? And the answer is, ‘Yes!’”