At one point not long ago, Black Panther was rumored to be the next solo superhero film to go into active development. Shortly thereafter, it was announced that Guardians of the Galaxy would be headed to theaters in 2014 and test footage from Ant-Man was shown at San Diego Comic-Con International, leading most fans to believe that character (also mentioned by Lee at Fan Expo) would be the next solo feature announced by the studio. Unlike Guardians of the Galaxy, for example, Ant-Man already has a director attached. A number of actors have expressed an interest in playing Black Panther in recent years.
Doctor Strange has long been considered a key to Marvel's "magical" universe, something that so far hasn't appeared on film. How he would fit with the current crop of films is unclear, but the character was a major part of Jim Starlin's The Infinity Gauntlet, the comics story upon which The Avengers 2 appears to be based, so don't be surprised if he pops up there and they attempt to move him forward if he catches on.
Of course, Lee also suggested that Deadpool (an X-Men property whose rights currently rest with Fox) could be reintroduced in a Captain America movie and that he believes they're working on a new Daredevil, so take all of this with a grain of salt: in spite of his many cameos in Marvel films, he may not be the best source of information.