Kevin Feige recently spoke about Scarlett Johansson's Black Widow role in the various upcoming Marvel films, and says that there's no plans to lower her participation. If anything, the talk of giving Natasha Romanoff her own feature film has increased:
"[Black] Widow's part in [Age of Ultron] is very big. We learn more about her past and learn more about where she came from and how she became in that film. The notion of exploring that even further in her own film would be great, and we have some development work with that."
Feige goes on to talk about the impact the upcoming "Captain America: The Winter Soldier" will have on "Avengers: Age of Ultron":
"We wanted to change the dynamic of the cinematic universe with this film. We wanted Cap and really the entire cinematic universe to be very different at the end of Winter Soldier than it is at the beginning.
Therefore when we meet the Avengers at the top of Age of Ultron, it's a very different landscape than we left them at the end of the film. Partially that's because we love the rhythm that the comic books have developed - each of the characters appear in their runs, occasionally they get together for a big event or crossover series, they part again, and then they come back together again."