Personally, I’m not in favor of integrating any sort of distraction free mode into WP. It’s hard enough to build a working UI for web design (as we can see with all the developments with Gutenberg since its launch). This would just add one more huge layer for bugs.

WordPress is not a writing tool. It’s a way to publish. To me, it would be like introducing a distraction free mode into InDesign. Do your writing in a tool designed for writing. When it’s ready, paste it into WP. I can’t fathom why anyone would do otherwise.