But in classical music, there is no broad outline on a relatively empty canvas. The picture is already complete. Every stroke, every color is already there, and it's the artist's job not to add or subtract, but to restore it and make it live again.
I don't disagree that every note is important. but at the same time classical music most definitely *is* constructed in a framework, and comes from a tradition where improvisation *was* important. This particular piece is obviously done that way, with the RH theme mutating throughought the piece in a very improvisatory manner. Chopin developed practically everything that way, he didn't sit down and compose at a desk like Beethoven in his later years.
I would be the first to admit I'm not worthy to presume to "improve" Chopin's efforts but let's not pretend that his pieces aren't composed in a very improvisatory manner.
I think I said earlier that notating chords would be less useful for contrapuntal works like Bach. You're absolute spot on there. I tried and it just doesn't work. Somewhat ironically though his work is renowned for its use of sequences and motifs -- elements that are considered in the jazz world to be important elements to a good solo. Bach and his son Carl Philip Emmanuel were legendary improvisers in their day, as were Chopin, Liszt and Beethoven.
As I said I personally am not worthy of "riffing on" the Nocturnes but I don't think it is stylistically inappropriate to do so. They all did. The idea that the sheet is holy writ from which one must not deviate is a fairly modern idea.