Seems to me there's a lot of improvisation goes on in folk music, very often fiddlers or whistle players, who maybe get to do an impromptu "fill" between verses of a song: those who are good at it, can really enhance a song and not just have to play the straight melody. (Bach might have called it "theme and variations")
We do it in my mixed instrument class, with some interesting and even quite melodic and harmonic results!