Besides the technical constraints (the zip code can't always tell you the city, etc), the zip code is more of a checksum to make sure the address you typed makes sense. I think you could probably get around the imperfections by making the city / state / zip be things suggested to you, whether in an autocomplete or by making them first in a dropdown. The imperfection of a zip code in encoding a city name doesn't mean that the frustration of filling out addresses isn't a real one!