I think the threads leading to Avengers 2 will come from within the Cap, Iron Man and Thor sequels. Probably a few actual superhero cameos, if they're cool and they make sense, and don't detract from the story like Fury did in IM2. If Thor has more than two scenes in Iron Man 3, for instance, it detracts from IM3, imho. But at the same time, the idea of Tony Stark's Hulkbuster appearing in TIH2 for a conference and a fight scene is awesome.
But at the same time, the new heroes: Black Panther, Ant-Man, Dr. Strange - they shouldn't have those cameos, because they need to be established as self sufficient.