Regarding Black Widow from the Q&A:
With Scarlett Johansson being one of the biggest movie stars on the planet, why did you decide not to give Black Widow her own movie?
Well, frankly, as I said in the presentation, it’s about bringing new characters to the screen. Black Widow couldn’t be more important as an Avenger, herself. Like Hulk, the Avengers films will be the films where they play a primary role. Her part in Avengers: Age of Ultron is very very big, and further develops and further enhances her character. The plans that we have for her throughout the rest of the Avengers saga is very very big, and lynchpin, in fact, to those films. So instead of taking her out there, or instead of doing a prequel, which we haven’t done yet, continuing the forward momentum and the continuity of the cinematic universe. Of which Widow is a key, key part.
Rest of it:
http://www.slashfilm.com/marvel-phase-3-kevin-feige/