According to Black, who announced there that he would be writing as well as directing, he'll be meeting with Robert Downey, Jr. this week, and that Downey will be contributing to the story.
Black pointed out that the studio was not pleased with the direction of Iron Man 2 and that the third film will not be "two men in iron suits fighting each other," but, instead, would play more like a Tom Clancy thriller, with Iron Man's opponents being more real-world than comic book.
Reportedly the film will not feature other Marvel heroes, because the plan is that post-Avengers the various films will focus on individual heroes, and that the crossovers were designed primarily as a lead in to that Joss Whedon-directed film.