I've just learned about the existence of the 2017 wikitext editor from the WikiProject RfC, and in the first 10 seconds of using it, I'm already missing the single feature I use more often in the classic wikitext editor: showing the page Preview and Changes alongside the text box with the Wikitext.
I've always missed in the Visual Editor that feature, and hated the fact that the "See changes" panel hides the final page below it; but, being visual, showing the Preview wasn't really needed. Not so with the Wikitext editor: here, hiding the page you were editing in order to see the final result is unacceptable.
Please review the workflow of the new editor so that it makes sense for the new use case you're introducing. It's merely a layout change (showing the preview panel above or below the edit page instead of floating and overlapping it), so I hope it shouldn't be exceedingly painful to program.