PDF and DocX files are open specifications that provide for extension. Nothing is stopping anyone from building clients around these formats with the features listed in the article. PDF is definitely more common as most programming languages have comprehensive libraries to work with it.
The path forward would to be build the features you want and publish the extension specifications for others to use. Perhaps the interesting question however isn't technical possibility but if a market exists for it? Email clients were very widespread over a decade ago but have consolidated to 3-4 over the years. Hey.com has been the first big new email clients that I am aware of. I'm curious if it can prove there is big business in improving on existing, standardized specifications.