You could probably break down every cue by every composer and search for influences. Sometimes the quote is more direct than others, though Williams isn't nearly as guilty of this as some other composers. The lush, romantic sound that I think of when I think of his music (Superman Love Theme, Harrys Wonderous World, etc.) is his own creation. I can't think of a piece of classical or film piece that used this sound before he had, the warm extended jazz chords used for a love theme. Of course I'm sure someone with encyclopedic knowledge of scores can prove me wrong here.