One of my favorite book cover designers is Peter Mendelsund. His competitive edge? As far as I can gather it’s that he deeply reads a text before he starts a design. Instead of bringing ideas to a text, his ideas come from the text. They are the source. And then he adds to it the rest of his education (literary, musical). Read a few direct quotes and further color here: http://www.npr.org/2014/10/16/345548582/the-jacket-designers-challenge-to-capture-a-book-by-its-cover
The main idea you posit in this piece — that if we can accept editing as a literary discipline then we can also accept graphic design as one — would find its best evidence of support in P. Mendelsund’s work. For example, his paperback version of Ulysses references a previous, well-loved design, while also uncovering a secret from the end of the book that you only get once you’ve finished it. He hides a spoiler in plain sight that offers a point of view — an editorial emphasis hierarchy–on what matters in the book. And what our experience is like as readers when we allow ourselves to be transformed by the text.