KISS please. Old School Sonic games never demanded that you go back and replay levels. Ergo, it makes more sense not to try and force players to have completed certain segments to access certain areas in previous levels. Provided that the levels are engaging and contain enough variety and choices on their own merit, having excess crap like that isn't necessary (and generally doesn't sound that exciting). Rather than directing your effort at making paths that open up with emeralds, spend that energy making more levels instead.