Black Widow a huge part of The Avengers 2

It appears that Joss Whedon is trying to make up for lost screen time.

After revealing that Hawkeye (Jeremy Renner) was going to have a bigger role in The Avengers: Age of Ultron, Whedon also elaborated on how Black Widow (Scarlett Johansson) will be a big part of the sequel as well. This comes after some criticism came after the two characters had very low screen time compared to the other heroes despite the fact that Black Widow had more screen time than both Thor and Hulk in The Avengers.
Hawkeye brought in a whopping 12:44 and Black Widow had 33:35. She brought in just 4min less screen time than Iron Man and Captain America and is a main focus in Captain America: The Winter Soldier, so I don't know how she can be an even bigger part in The Avengers: Age of Ultron unless she surpasses the other heroes and becomes the absolute main character.

Also added for Age of Ultron are Quicksilver and Scarlet Witch, the twin children of Magneto. Whedon expressed his fun with Black Widow due to the fact that she is just a spy who doesn't have the same morality the others possess.

The Avengers: Age of Ultron not based off comic arc

Or so Joss Whedon says. Now bear in mind this is the same guy who assured us Iron Man 3 wouldn't have the Iron Patriot, so I take what he says with a grain of salt.

But on with it... Joss Whedon has said that the upcoming Avengers sequel, The Avengers: Age of Ultron, will not be based off the Age of Ultron comic arc where Wolverine and the Invisible Woman are sent into the past to kill Ant-Man so he can't create Ultron. Whedon assured us that they are working on an original interpretation of the origin of Ultron story. He basically also said that people should stop thinking Ant-Man (aka, Henry Pym) will make a cameo in The Avengers 2 as a lead into Edgar Wright's Ant-Man movie.

Other than that he mentions that Age of Ultron will feature Hawkeye in a bigger, more important role and we will see him as more than the possessed scapegoat he was in The Avengers. Age of Ultron will also be a more global film in regards to the team not only worrying about the United States but about the fate of Earth and the whole galaxy.

The Avengers: Age of Ultron opens May 1, 2015 and will be followed by Ant-Man on November 6, 2015.