Well, it looks as though you can add Black Widow to that list as well, with the filmmaker saying that Avengers: Age of Ultron will feature Scarlett Johansson's character prominently.
"Natasha is a huge part of the sequel because you do want to concentrate on the people who don't have their own franchises," Whedon said in the interview you can watch below. "Although she in [Captain America: The Winter Soldier], [and] she's great. She was the most fun for me because she's not a hero, you know, and it's something that I read—and I feel bad that I can't remember who wrote the book—but it's in one of the books explaining, 'These guys are heroes, you are a spy. It's a different thing—it's a different skill set—and you don't have their moral high ground or any of that good stuff.' And that just makes her so interesting to me. So yeah, the stuff I've got going on with her in the second one is killer."
Her increased role will have to share some screen time with new characters; in addition to introducing the threat of Ultron, who will presumably demand more screen time than did Loki and the Chitauri threat to establish, the film is expected to bring Quicksilver and Scarlet Witch to the Marvel Studios fold.