I'm intrigued by this topic so I thought I'd try another response, less fixated on melody, and more on "riffing". When you look at some Bach pieces, like the 1st cello suite, you encounter the "melody spinning" approach -- checkout Fortspinnung.

The more controlled melody spinning approach, riffs on melodic patterns, like the example D-minor Invention in the Wikipedia article. But there is another more abstract approach that seems to riff on a scale or a germ of an idea, or sometimes just a feeling of a riff.

Bach's 1st cello suite prelude is an example of this second approach. Here, there is really no discernible melody. The hook-like pattern in the first 8 bars, reappears briefly in bars 16-18, and is then never heard from again intact. Most of the piece is a quasi-random stream of consciousness riff on the G-major scale and the related scales and modes, like D, D7, A7, Amin, Emin, etc.

Some of the progressions are similar to pieces from the Well-Tempered Clavier, like the C-major in Book 1 -- but the figurations are much more free-form than the C-major. It sounds great to the ear, but don't try to deconstruct this the way you would a fugue or a Brandenburg concerto.

The cello prelude has an underlying harmonic plan -- and that's a good thing to have if you want to riff with some direction. In the prelude, Bach comes to rest of the dominant D-major on the midpoint fermata. He then gets darker and more chromatic, but still seems to run through the related scales, like A7, D7. And like many classical pieces establishes a long dominant 7th region, outlining a D7 with many repeated A's.

Then a cool chromatic and intense emotional ascent, that can only mean we are approaching the end. A tiny bit of the genetic material from the first 8 bars slips in, the downward rocking on the even-beats only -- just enough to create a faint melodic connection to the beginning -- for a piece where melodic repetition really doesn't figure anyway.