Raga, Maqam, ... all sorts really. A couple of hundred years of western music is a non-improvising exception.
Well...the fact that you think that needs to be said implies a misunderstanding. There is no judgment here about quality of music, depending on whether improvisation is part of it, *essentially*. But improvisation is the essence of jazz. If you don't understand that, you don't understand jazz.
As above, pretty much every canonical giant in jazz loved non-improvisatory music, and non-improvisatory music greatly informs their playing -- most found numerous Western classical composers deeply formative. But that doesn't make jazz non-improvisatory and, again, any one who does not understand that improvisation is what makes "the idiom" itself, that that is the most fundamental aspect, does not know the single most important thing about it. For that not to be obvious is surreally absurd. It means you don't "get it." At all.
It's also fundamentally a music of resistance, not just because of that improvisatory basis, and how it plugs into the society in which the music arose and against whose social ideologies its aesthetic practices were aimed, and much of the canonically defining inspiration was formed in full knowledge of current audiences' disapproval of innovation (because people want both cultural productions, and culture, to remain the same). If you don't understand that, you also don't understand jazz. You could love it, but you don't understand it.
The people that play jazz, as demographic constituencies, has shifted massively over time, and because that change is largely toward a more technical, less "social" (in the way Miles likely intended when he said of his music, objecting to the term jazz, "Don't call it that...call it social music") music, the social demographic of practitioners changing (to be more white, more middle-class and affluent, more bourgeois) is probably only going to change even more in that direction, but specifically to include more people on the autism spectrum, because that's who enjoys sitting in a room alone, doing something exactly the same way over and over again. Technical musicians in the past did that because they had a drive separate from rote practice (doing something over and over again, exactly the same way is a rote practice) *causing* them discipline themselves to drill scales endlessly -- there was a reward at the end of it, beauty, separate from the rote practice itself, driving the will to discipline oneself. Ask a person on the bipolar spectrum how they feel about doing scales, especially if they have to do them "more or less" the same way over and over -- it sounds torture to a person with an overactive lymbic system (bipolarism) which is probably a huge swath of our favorite jazz musicians of the past; they need to have OCD or ADHD to do that, and even then they need a reward down the line, something to make them willing (or even eager) to subject themselves to a regime of torture in order to ascend into joy. Autism spectrum practitioners find a reward in the repetition itself. That is already changing the music, and it's going to change it more.
There are people well into "the spectrum" that improvise joyfully and freely. There are others (probably Glenn Gould is an example) who are generally horrified by everything the sound of unplanned, potentially random/surprising improvisation itself creates.
That already has severe consequences for the music that are only going to compound over time. The people that created the canons probably wouldn't be in jazz today because hiphop has a lower technical/repetition/boredom requirement at every level of production. What is that, hip hop, except today's "social music," after all.
Someone will probably misunderstand and think I'm bagging on people with Aspergers and/or "on the spectrum." I'm not. At the highest level, today, I think most of the players are on the spectrum, because the technical demands today are so ridiculously high. I also think (and hear) that a new kind of beauty and level of aesthetic beauty -- by which I mean expression of human spirit and emotion -- comes out of players who are at that level of playing, the kind of players you hear killing it on "Emmett's Place." To me, with certain combinations of players, the players are at such a high level that they are relating to each other *through* the text, rather than the other way around, which I think was the default norm in the past. In Coltrane's saying, "When I hear a man's sound, to me, that IS that man," I take him to mean he is looking for the soul of man (for him, of God) and he seeks it in the music -- but his focus is on the man, and being able to hear him, inevitably hearing him, in his sound. I think many of today's players at that really, really high level -- maybe most at that level, though they may disagree -- don't seek in that same way. They hear the man in the patterns themselves. The patterns are the man. Neither is better or worse, objectively, but they are different.
That's going to have a lot of consequences for the music and culture (both American and globally). It already has. It's a big deal, that reminds a person that cultural productions are always of the cultures of their times, even when they are specifically trying not to be.