Is it possible that the REAL underlying problem is the the_content approach to storing document data? I believe that problem should be solved first. Imagine a future where the_content is pulled from its own API that can optionally be stored in a noSQL database. A hybrid db approach. So continue to use MySQL, but use a noSQL db just for post/page/cpt content. Then allow such an API to interact with that instead. Just my 2 cents.