Shield always monitored other heroes and Fury didn't mean anyone specific. It was just an easter egg to let fans know more movies would be coming. Dr. Strange, Moon Knight, Black Panther, Spider-Man, The Inhumans royal family ect. Could've all existed already. We were shown Iron man, Hulk, Thor, and Cap because they are the Avengers and the first franchise characters Marvel was planning so they had to be introduced first.