Impressionism of developing the whole tone scale and harmonic ambiguity creates a beautiful blurred atmosphere of peaceful seas/clouds etc. This really could have been excellent innovation of a slow movement within a symphony/concerto/sonata.
After the intellectual first sonata form movement where there is high degree of harmonic turnover. The slow second movement of impressionism and harmonic ambiguity would serve an excellent respite and contrast to the opening movement. In a way this could be the French version of symphonic or sonata structure (similar to my previous thought of Stravinsky rhythmic development in Firebird could have been the russian version of sonata form)
Alas Debussy decided that impressionism could sustain itself as a singular genre of music that he specialised in which is a real shame.
when I listen to Prélude à l'après-midi d'un faune, I just think this would make a great slow movement of a larger symphony instead of an indepedent piece of music.