Captain America: The Winter Soldier seems to confirm the return of Sebastian Stan's Bucky Barnes in the sequel. In the comic books, Bucky nearly dies in a plane explosion over Moscow, where scientists attach a bionic arm and program him as an assassin. He also had a brief relationship with The Black Widow in another comic book story arc. As for Thor: The Dark World, we're not quite sure what this "dark world" really is.
In related Marvel news, it was also confirmed that Ben Kingsley is in fact playing The Mandarin in Iron Man 3. We reported back in April that the actor signed on, although insiders were adamant that he was not playing this particular villain. Director Shane Black was also present at the panel, where he revealed that they are halfway through principal photography.