Elements of non-functional harmony is ubiquitous among contemporary music creations. In most cases, it materializes in the way of non-functional chord progressions, which are characterized by its "successive" feel and lack of diatonic association, in contrast to the "progressive" feel functional harmony emits, which is the natural result of attempting to establish a tonal center via resolutions, often accompanied by a bass movement of perfect fifths.
A majority of music theory books pertains to Western music during the Common Practice Period refrain from discussing the topic of non-functional harmony (though some do talk about atonal theory and the like), and those do often focus on enharmonic chords, (tritone) substitutions and deceptive cadences, or are sometimes presented only through short excerpts, which do not give much insights in the matter.
However, some successive chord progressions can still be analyzed in the traditional and functional way. For example, the chord progression
FM7 - Fm7 - Fm6 - C in the key of C Major can be thought as a Plagal Cadence extended via mode mixture, and has two voice leading lines:
6 - b6 - 5 and
3 - b3 - 2 - 1 that contributes to a strong cadence. The
ii - V progression can be enhanced by throwing in a secondary dominant chord:
ii - V/V - V, creating a smooth voice leading of
4 - #4 - 5.
In fact, some functional chord progressions, especially cadences, can be regarded as successive, because they contain (sometimes chromatic) leading tones that naturally resolve to a diatonic note. However, more often than not are successive motions in modern music - like ascending or descending bass lines - non-functional.
To give a few examples:
The first few chords of the intro to Led Zeppelin's Stairway to Heaven form a chord progression of
Am - E/G# - C/G - D/F# - Fmaj7, which has a chromatic descending bass line of
6 - #5 - 5 - #4 - 4. If you try to analyze it the functional way, then maybe the first two chords can be thought as a retrogression
i - V6/i, and maybe the next C/G can be regarded as a deceptive cadence, but then it stops there.
This electronic music track of Art Of Trance - Blue Owl has a progression of
Cm7 - F7 - AbMaj6 - Cm7/G starting from 5:24, which in Common Practice Roman Numerals is
i7 - IV7 - iv56 - i46, producing a voice leading of
5 - #4 - 4 and a nice chromatic feeling.
The above two pieces both have their distinct tonal centers, but sometimes even those are derived from non-functional harmony. The intro to the Glitch Hop piece Haywyre - Doppelgänger features a progression of
Fm7 - Gm7 - GbMaj7 - Bbm7 - F/A - Fm/Ab - DbMaj7 - GbMaj9, which already doesn't make much sense when written in Roman Numerals:
i7 - ii7 - bII7 - iv7 - I6 - i6 - VI7 - bII9, but one can identify the three chromatic voice leading successions:
5 - #4 - 4 - 3 (starting from Fm7),
1 - 7 - b7 - 6 (starting from Fm7) and
2 - #1 - 1 (starting from Bbm7). Also, the ambiguity with regard to its tonal center certainly contributes to its good vibe.
From all the examples above we can find out that chromaticism and smooth voice leading plays a big role in successive, non-functional harmony, but simply throwing a bunch of chords that share chromatic passing notes together just won't work. Some books discussing voice-leading introduced a technique of judging how far two temporally adjacent chords are away in a voice-leading sense by counting the semitones needed to change a chord into the other (sort of like earth mover's distance), which made me think that the same technique could be used in writing successive chord progressions, but apparently it isn't that simple, as some chord notes can still come as unprepared when the chord plays.
Which brings me to my question, how DO you write successive, non-functional chord progressions like chromatic ascending/descending bass? Do you play on the keyboard and hope something good will arise through trial and error? Are there any rules or advanced music theory topics regarding how to write it? Does it just come by naturally during the process of improvisation? Or is it a talent that someone is just good at without reason?
P.S. I really tried to keep the question from being overly subjective, but I don't know if that's possible.