Rich can easily be given with an example rather than purely aesthetics. Coltrane introduced more advanced harmonic movement via "Coltrane changes" as one example. It had been done in one-off cases in classical music (like just about anything), and occasionally in show tunes (Have You Met Miss Jones) but Coltrane was the first to isolate the specific pattern and make it into a reusable substitution technique to reharmonize other tunes.

Aside from that, Coltrane's contribution to modal jazz is significant. Though not singularly from him (or anyone else), he contributed quite a few standards and recordings to a style that was a pretty radical and rich departure from functional harmony. And if you don't consider the harmonic system this resides in (Russell's LCC is a common one used to analyze it) as "rich" then I think you might be too harsh a critic.